The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.
This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:
Refer to thecolophonfor more information on how this document is created.
Introduction
This Ecma Standard defines the ECMAScript 2022 Language. It is the twelfth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplemental characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a newmemory modelthat allows multi-agentprograms to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: Object.values, Object.entries, and Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduces the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, ahost-populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (null or undefined): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduces the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in animplementation-definedsort order.
Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.
Allen Wirfs-Brock ECMA-262, Project Editor, 6th Edition
Brian Terlson ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband ECMA-262, Project Editor, 10th through 12th Editions
1 Scope
This Standard defines the ECMAScript 2022 general-purpose programming language.
2 Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause12.6.2of this specification.
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause17.1.
A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.
A conforming implementation of ECMAScript must implement Legacy subclauses, unless they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in existing applications prevents their removal from this specification. These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.
2.3 Example Legacy Normative Optional Clause Heading
Example clause contents.
3 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 10646 Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within ahost environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide ahost environmentof objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety ofhostenvironments. Therefore the core language is specified in this document apart from any particularhost environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScripthost environmentfor client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, thehost environmentprovides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction, and there is no need for a main program.
A web server provides a differenthost environmentfor server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its ownhost environment, completing the ECMAScript execution environment.
4.2 Hosts and Implementations
To aid integrating ECMAScript intohostenvironments, this specification defers the definition of certain facilities (e.g.,abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities enumerated in AnnexDor those that are marked asimplementation-definedorimplementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.
An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such asMath.exp, areimplementation-approximated.
A host is an external source that further defines facilities listed in AnnexDbut does not further define otherimplementation-definedorimplementation-approximatedfacilities. In informal use, ahostrefers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via AnnexD. Ahostis often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that arehost-definedare often further defined in external specifications.
A host hook is an abstract operation that is defined in whole or in part by an external source. Allhosthooks must be listed in AnnexD. Ahost hookmust conform to at least the following requirements:
A host-defined facility is one that defers its definition to an external source without further qualification and is listed in AnnexD. Implementations that are not hosts may also provide definitions forhost-definedfacilities.
A host environment is a particular choice of definition for allhost-definedfacilities. Ahost environmenttypically includes objects or functions which allow obtaining input and providing output ashost-definedproperties of theglobal object.
This specification follows the editorial convention of always using the most specific term. For example, if a facility ishost-defined, it should not be referred to asimplementation-defined.
Both hosts and implementations may interface with this specification via the language types, specification types,abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.
ECMAScript is object-based: basic language andhostfacilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set tofalse, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include theglobal object; objects that are fundamental to theruntime semanticsof the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Eachconstructoris a function that has a property named"prototype"that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009, 11) creates a new Date object. Invoking aconstructorwithout using new has consequences that depend on theconstructor. For example, Date() produces a string representation of the current date and time rather than an object.
Every object created by aconstructorhas an implicit reference (called the object's prototype) to the value of itsconstructor's"prototype"property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:
CF is aconstructor(and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named"q1"and"q2". The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. Theconstructor, CF, has two properties itself, named"P1"and"P2", which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named"CFP1"in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named"q1","q2", or"CFP1". Notice that there is no implicit prototype link between CF and CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern ofconstructorfunctions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in11.2.2. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
animplementation-approximatedfacility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this specification
4.4.2 implementation-defined
animplementation-definedfacility is defined in whole or in part by an external source to this specification
The value of aconstructor's"prototype"property is a prototype object that is used to implement inheritance and shared properties.
4.4.8 prototype
object that provides shared properties for other objects
Note
When aconstructorcreates an object, that object implicitly references theconstructor's"prototype"property for the purpose of resolving property references. Theconstructor's"prototype"property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
object whose semantics are defined by this specification
4.4.12 built-in object
object specified and supplied by an ECMAScript implementation
Note
Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-inconstructor is a built-in object that is also aconstructor.
4.4.13 undefined value
primitive value used when a variable has not been assigned a value
4.4.14 Undefined type
type whose sole value is theundefinedvalue
4.4.15 null value
primitive value that represents the intentional absence of any object value
4.4.16 Null type
type whose sole value is thenullvalue
4.4.17 Boolean value
member of the Boolean type
Note
There are only two Boolean values,trueandfalse.
4.4.18 Boolean type
type consisting of the primitive valuestrueandfalse
4.4.19 Boolean object
member of the Object type that is an instance of the standard built-in Booleanconstructor
Note
A Boolean object is created by using the Booleanconstructorin a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsignedintegervalues
Note
A String value is a member of the String type. Eachintegervalue in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object type that is an instance of the standard built-in Stringconstructor
Note
A String object is created by using the Stringconstructorin a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the Stringconstructoras a function (22.1.1.1).
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary formatIEEE 754-2019value
Note
ANumber valueis a member of the Number type and is a direct representation of a number.
4.4.24 Number type
set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity
4.4.25 Number object
member of the Object type that is an instance of the standard built-in Numberconstructor
Note
A Number object is created by using the Numberconstructorin a new expression, supplying aNumber valueas an argument. The resulting object has an internal slot whose value is theNumber value. A Number object can be coerced to aNumber valueby calling the Numberconstructoras a function (21.1.1.1).
primitive value corresponding to an arbitrary-precisionintegervalue
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object type that is an instance of the standard built-in BigIntconstructor
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object type that is an instance of the standard built-in Symbolconstructor
4.4.34 function
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.
4.4.35 built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. Ahostor implementation may provide additional built-in functions that are not described in this specification.
4.4.36 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or afunction object) or indirectly by a pair of accessor functions.
4.4.37 method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function as itsthisvalue.
4.4.38 built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification. Ahostor implementation may provide additional built-in methods that are not described in this specification.
4.4.39 attribute
internal value that defines some characteristic of a property
4.4.40 own property
property that is directly contained by its object
4.4.41 inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause5defines the notational conventions used throughout the specification.
Clauses6through10define the execution environment within which ECMAScript programs operate.
Clauses11through17define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.
Clauses18through28define the ECMAScript standard library. They include the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.
Clause29describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (12.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. AMultiLineComment(that is, a comment of the form /*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if aMultiLineCommentcontains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in22.2.1. This grammar also has as its terminal symbols the code points as defined bySourceCharacter. It defines a set of productions, starting from thegoal symbolPattern, that describe how sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.
5.1.3 The Numeric String Grammar
Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbolsSourceCharacter. This grammar appears in7.1.4.1.
Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses13through16. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbolsScriptandModule, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScriptScriptorModule, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (ScriptorModule), with no tokens left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse'sgoal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into an ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses13through16is not a complete account of which token sequences are accepted as a correct ECMAScriptScriptorModule. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScriptScriptorModule. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. Typically, anearly errorrule will then define an error condition if "P is not covering an N", where P is a Parse Node (an instance of the generalized production) and N is a nonterminal from the supplemental grammar. Here, the sequence of tokens originally matched by P is parsed again using N as thegoal symbol. (If N takes grammatical parameters, then they are set to the same values used when P was originally parsed.) An error occurs if the sequence of tokens cannot be parsed as a single instance of N, with no tokens left over. Subsequently, algorithms access the result of the parse using a phrase of the form "the N that is covered by P". This will always be a Parse Node (an instance of N, unique for a given P), since any parsing failure would have been detected by anearly errorrule.
5.1.5 Grammar Notation
Terminal symbols are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \UnicodeEscapeSequence.
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:
states that the nonterminalWhileStatementrepresents the token while, followed by a left parenthesis token, followed by anExpression, followed by a right parenthesis token, followed by aStatement. The occurrences ofExpressionandStatementare themselves nonterminals. As another example, the syntactic definition:
states that anArgumentListmay represent either a singleAssignmentExpressionor anArgumentList, followed by a comma, followed by anAssignmentExpression. This definition ofArgumentListis recursive, that is, it is defined in terms of itself. The result is that anArgumentListmay contain any positive number of arguments, separated by commas, where each argument expression is anAssignmentExpression. Such recursive definitions of nonterminals are common.
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:
so, in this example, the nonterminalForStatementactually has four alternative right-hand sides.
A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.
If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite nonempty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.
matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexicalgoal symbolto use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.
If the phrase “[noLineTerminatorhere]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if aLineTerminatoroccurs in the input stream at the indicated position. For example, the production:
indicates that the production may not be used if aLineTerminatoroccurs in the script between the throw token and theExpression.
Unless the presence of aLineTerminatoris forbidden by a restricted production, any number of occurrences ofLineTerminatormay appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.
When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminalIdentifiermay be replaced by any sequence of code points that could replaceIdentifierNameprovided that the same sequence of code points could not replaceReservedWord.
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into aList. If there are no such additional arguments, thatListis empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps1,3, and4in the following algorithm:
Let status be SyntaxDirectedOperation ofSomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode passing"value"as the argument.
Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to thechain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return the result of evaluatingBlock” and that there is a production:
but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:
Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined byabstract operationsor syntax-directed operations. Such algorithms always return a completion record.
5.2.3.1 Implicit Completion Values
The algorithms of this specification often implicitly returnCompletionRecords whose [[Type]] isnormal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not aCompletion Record, such as:
However, if the value expression of a “return” statement is aCompletion Recordconstruction literal, the resultingCompletion Recordis returned. If the value expression is a call to an abstract operation, the “return” statement simply returns theCompletion Recordproduced by the abstract operation.
The abstract operationCompletion(completionRecord) is used to emphasize that a previously computedCompletion Recordis being returned. TheCompletionabstract operation takes a single argument, completionRecord, and performs the following steps:
Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return anabrupt completionand that the resultingCompletion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:
Syntax-directed operations forruntime semanticsmake use of this shorthand by placing ! or ? before the invocation of the operation:
Perform ! SyntaxDirectedOperation ofNonTerminal.
5.2.4 Static Semantics
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScriptScriptorModulethat may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
A special kind of static semantic rule is an Early Error Rule.Early errorrules defineearly errorconditions (see clause17) that are associated with specific grammar productions. Evaluation of mostearly errorrules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of aScriptorModule, validate all of theearly errorrules of the productions used to parse thatScriptorModule. If any of theearly errorrules are violated theScriptorModuleis invalid and cannot be evaluated.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
Mathematical values: Arbitrary real numbers, used as the default numeric type.
Extended mathematical values: Mathematical values together with +∞ and -∞.
Numbers:IEEE 754-2019double-precision floating point values.
BigInts: ECMAScript values representing arbitrary integers in a one-to-one correspondence.
In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical values.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to Numbers, the operators refer to the relevant operations withinIEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to themathematical valueof the BigInt.
In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "theintegerrepresented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to amathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "theNumber valuefor the number of code points in …" or "the BigInt value for …".
Numeric operators applied to mixed-type operands (such as a Number and amathematical value) are not defined and should be considered an editorial error in this specification.
This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.
When the term integer is used in this specification, it refers to amathematical valuewhich is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to aNumber valuewhosemathematical valueis in the set of integers.
Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from amathematical valueorextended mathematical valuex to a Number is denoted as "theNumber valuefor x" or 𝔽(x), and is defined in6.1.6.1. A conversion from anintegerx to a BigInt is denoted as "the BigInt value for x" or ℤ(x). A conversion from a Number or BigInt x to amathematical valueis denoted as "the mathematical value of x", or ℝ(x). Themathematical valueof+0𝔽 and-0𝔽 is themathematical value0. Themathematical valueof non-finite values is not defined. The extended mathematical value of x is themathematical valueof x for finite values, and is +∞ and -∞ for+∞𝔽 and-∞𝔽 respectively; it is not defined forNaN.
The mathematical functionabs(x)produces the absolute value of x, which is-xif x < 0 and otherwise is x itself.
The mathematical functionmin(x1, x2, … , xN)produces the mathematically smallest ofx1throughxN. The mathematical functionmax(x1, x2, ..., xN)produces the mathematically largest ofx1throughxN. The domain and range of these mathematical functions are the extended mathematical values.
The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such thatabs(k) <abs(y) and x - k = q × yfor someintegerq.
The phrase "the result of clamping x between lower and upper" (where x is anextended mathematical valueand lower and upper are mathematical values such that lower ≤ upper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.
The mathematical functionfloor(x)produces the largestinteger(closest to +∞) that is not larger than x.
Mathematical functions min, max,abs, andfloorare not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical valuearguments would be an editorial error in this specification.
In this specification, ECMAScript language values are displayed inbold. Examples includenull,true, or"hello". These are distinguished from longer ECMAScript code sequences such as Function.prototype.apply or let n = 42;.
Values which are internal to the specification and not directly observable from ECMAScript code are indicated with asans-seriftypeface. For instance, aCompletion Record's [[Type]] field takes on values likenormal,return, orthrow.
6 ECMAScript Data Types and Values
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, calledundefined. Any variable that has not been assigned a value has the valueundefined.
6.1.2 The Null Type
The Null type has exactly one value, callednull.
6.1.3 The Boolean Type
The Boolean type represents a logical entity having two values, calledtrueandfalse.
6.1.4 The String Type
The String type is the set of all ordered sequences of zero or more 16-bit unsignedintegervalues (“elements”) up to a maximum length of 253 - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.
ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive range 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive range 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:
A sequence of two code units, where the first code unit c1 is aleading surrogateand the second code unit c2 atrailing surrogate, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See11.1.3)
The function String.prototype.normalize (see22.1.3.13) can be used to explicitly normalize a String value. String.prototype.localeCompare (see22.1.3.10) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results.
Note
The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.
The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negativeinteger). It performs the following steps when called:
If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
Let searchLen be the length of searchValue.
For eachintegeri starting with fromIndex such that i ≤ len - searchLen, in ascending order, do
Let candidate be thesubstringof string from i to i + searchLen.
If candidate is the same sequence of code units as searchValue, return i.
Return -1.
Note 1
If searchValue is the empty String and fromIndex is less than or equal to the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.
Note 2
This algorithm always returns -1 if fromIndex > the length of string.
6.1.5 The Symbol Type
The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that is eitherundefinedor a String value.
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (9.3).
Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed inTable 1.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
@@asyncIterator
"Symbol.asyncIterator"
A method that returns the default AsyncIterator for an object. Called by the semantics of the for-await-of statement.
@@hasInstance
"Symbol.hasInstance"
A method that determines if aconstructorobject recognizes an object as one of theconstructor's instances. Called by the semantics of the instanceof operator.
@@isConcatSpreadable
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array elements byArray.prototype.concat.
@@iterator
"Symbol.iterator"
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by theString.prototype.matchmethod.
@@matchAll
"Symbol.matchAll"
A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by theString.prototype.matchAllmethod.
@@replace
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by theString.prototype.replacemethod.
@@search
"Symbol.search"
A regular expression method that returns the index within a string that matches the regular expression. Called by theString.prototype.searchmethod.
@@species
"Symbol.species"
A function valued property that is theconstructorfunction that is used to create derived objects.
@@split
"Symbol.split"
A regular expression method that splits a string at the indices that match the regular expression. Called by theString.prototype.splitmethod.
@@toPrimitive
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by theToPrimitiveabstract operation.
@@toStringTag
"Symbol.toStringTag"
A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in methodObject.prototype.toString.
@@unscopables
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. In this specification, every numeric type T contains a multiplicative identity value denoted T::unit. The specification types also have the followingabstract operations, likewise denoted T::op for a given operation with specification name op. All argument types are T. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return anabrupt completion.
The T::unit value and T::op operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Otherabstract operationsare defined throughout this specification.
Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.
6.1.6.1 The Number Type
The Number type has exactly 18,437,736,874,454,810,627 (that is,264 - 253 + 3) values, representing the double-precision 64-bit formatIEEE 754-2019values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is,253 - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single specialNaNvalue. (Note that theNaNvalue is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour isimplementation-defined; to ECMAScript code, allNaNvalues are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see25.1) or a SharedArrayBuffer (see25.2) after aNumber valuehas been stored into it is not necessarily the same as the internal representation of thatNumber valueused by the ECMAScript implementation.
There are two other special values, calledpositive Infinityandnegative Infinity. For brevity, these values are also referred to for expository purposes by the symbols+∞𝔽 and-∞𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is,264 - 253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positiveNumber valuethere is a corresponding negative value having the same magnitude.
Note that there is both apositive zeroand anegative zero. For brevity, these values are also referred to for expository purposes by the symbols+0𝔽 and-0𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)
The 18,437,736,874,454,810,622 (that is,264 - 253 - 2) finite non-zero values are of two kinds:
18,428,729,675,200,069,632 (that is,264 - 254) of them are normalized, having the form
s × m × 2e
where s is 1 or -1, m is anintegersuch that 252 ≤ m < 253, and e is anintegersuch that -1074 ≤ e ≤ 971.
The remaining 9,007,199,254,740,990 (that is,253 - 2) values are denormalized, having the form
s × m × 2e
where s is 1 or -1, m is anintegersuch that 0 < m < 252, and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type. Theinteger0 has two representations in the Number type:+0𝔽 and-0𝔽.
A finite number has an odd significand if it is non-zero and theintegerm used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means aNumber valuechosen in the following manner. Consider the set of all finite values of the Number type, with-0𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is+1 × 253 × 2971) and-21024(which is-1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and-21024are considered to have even significands. Finally, if 21024 was chosen, replace it with+∞𝔽; if-21024was chosen, replace it with-∞𝔽; if+0𝔽 was chosen, replace it with-0𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is theNumber valuefor x. (This procedure corresponds exactly to the behaviour of theIEEE 754-2019roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific ranges such as-231through231 - 1, inclusive, or in the range 0 through216 - 1, inclusive. These operators accept any value of the Number type but first convert each such value to anintegervalue in the expected range. See the descriptions of the numeric conversion operations in7.1.
The Number::unit value is1𝔽.
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number). It performs the following steps when called:
If x isNaN, returnNaN.
Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number). It performs the following steps when called:
Return the result of applying bitwise complement to oldValue. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base, exponent )
The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number). It returns animplementation-approximatedvalue representing the result of raising base to the exponent power. It performs the following steps when called:
If exponent isNaN, returnNaN.
If exponent is+0𝔽 or exponent is-0𝔽, return1𝔽.
If base isNaN, returnNaN.
If base is+∞𝔽, then
If exponent >+0𝔽, return+∞𝔽. Otherwise, return+0𝔽.
If base is-∞𝔽, then
If exponent >+0𝔽, then
If exponent is an oddintegral Number, return-∞𝔽. Otherwise, return+∞𝔽.
Else,
If exponent is an oddintegral Number, return-0𝔽. Otherwise, return+0𝔽.
If base is+0𝔽, then
If exponent >+0𝔽, return+0𝔽. Otherwise, return+∞𝔽.
If base is-0𝔽, then
If exponent >+0𝔽, then
If exponent is an oddintegral Number, return-0𝔽. Otherwise, return+0𝔽.
Else,
If exponent is an oddintegral Number, return-∞𝔽. Otherwise, return+∞𝔽.
The result of base**exponent when base is1𝔽 or-1𝔽 and exponent is+∞𝔽 or-∞𝔽, or when base is1𝔽 and exponent isNaN, differs fromIEEE 754-2019. The first edition of ECMAScript specified a result ofNaNfor this operation, whereas later versions ofIEEE 754-2019specified1𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a Number). It performs multiplication according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:
Finite-precision multiplication is commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and y (a Number). It performs division according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:
If x isNaNor y isNaN, returnNaN.
If x is+∞𝔽 or x is-∞𝔽, then
If y is+∞𝔽 or y is-∞𝔽, returnNaN.
If y is+0𝔽 or y >+0𝔽, return x.
Return -x.
If y is+∞𝔽, then
If x is+0𝔽 or x >+0𝔽, return+0𝔽. Otherwise, return-0𝔽.
If y is-∞𝔽, then
If x is+0𝔽 or x >+0𝔽, return-0𝔽. Otherwise, return+0𝔽.
The abstract operation Number::remainder takes arguments n (a Number) and d (a Number). It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:
Let r beℝ(n) - (ℝ(d) × q) where q is anintegerthat is negative if and only if n and d have opposite sign, and whose magnitude is as large as possible without exceeding the magnitude ofℝ(n) /ℝ(d).
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
Note 2
The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined byIEEE 754-2019. TheIEEE 754-2019“remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usualintegerremainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Javaintegerremainder operator; this may be compared with the C library function fmod.
6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number). It performs addition according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:
Finite-precision addition is commutative, but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a Number). It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:
Return the result of left shifting lnum by shiftCount bits. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x, y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x, y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. Themathematical valueof the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number). It performs the following steps when called:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1 ≤ s < 10k,𝔽(s × 10n - k) is x, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
The least significant digit of s is not always uniquely determined by the requirements listed in step5.
Note 2
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step5be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1 ≤ s < 10k,𝔽(s × 10n - k) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n - k is closest in value toℝ(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
The BigInt type represents anintegervalue. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.
The BigInt::unit value is1ℤ.
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt). It performs the following steps when called:
If x is0ℤ, return0ℤ.
Return the BigInt value that represents the negation ofℝ(x).
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt). It returns the one's complement of x; that is, -x -1ℤ.
6.1.6.2.3 BigInt::exponentiate ( base, exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt). It performs the following steps when called:
If exponent <0ℤ, throw aRangeErrorexception.
If base is0ℤ and exponent is0ℤ, return1ℤ.
Return the BigInt value that representsℝ(base) raised to the powerℝ(exponent).
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the result of multiplying x and y.
Note
Even if the result has a much larger bit width than the input, the exact mathematical answer is given.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
Return the BigInt value that represents quotient rounded towards 0 to the nextintegervalue.
6.1.6.2.6 BigInt::remainder ( n, d )
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt). It performs the following steps when called:
If d is0ℤ, throw aRangeErrorexception.
If n is0ℤ, return0ℤ.
Let r be the BigInt defined by the mathematical relation r = n - (d × q) where q is a BigInt that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.
Return r.
Note
The sign of the result equals the sign of the dividend.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the sum of x and y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the difference x minus y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
If y <0ℤ, then
Return the BigInt value that representsℝ(x) / 2-y, rounding down to the nearestinteger, including for negative numbers.
The abstract operation BigIntBitwiseOp takes arguments op (a sequence of Unicode code points), x (a BigInt), and y (a BigInt). It performs the following steps when called:
Return the String value consisting of the code units of the digits of the decimal representation of x.
6.1.7 The Object Type
An Object is logically a collection of properties. Each property is either a data property, or an accessor property:
A data property associates a key value with anECMAScript language valueand a set of Boolean attributes.
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve anECMAScript language valuethat is associated with the property.
Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.
An integer index is a String-valued property key that is a canonical numeric String (see7.1.21) and whose numeric value is either+0𝔽 or a positiveintegral Number≤𝔽(253 - 1). An array index is aninteger indexwhose numeric value i is in the range+0𝔽 ≤ i <𝔽(232 - 1).
Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see6.1.7.2for definitions of the multiple forms of objects.
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object properties. Adata propertyassociates a key value with the attributes listed inTable 3.
The value retrieved by a get access of the property.
[[Writable]]
Boolean
Iffalse, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]
Boolean
Iftrue, the property will be enumerated by a for-in enumeration (see14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
Iffalse, attempts to delete the property, change the property to be anaccessor property, or change its attributes (other than [[Value]], or changing [[Writable]] tofalse) will fail.
If the value is an Object it must be afunction object. The function's [[Call]] internal method (Table 7) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]
Object | Undefined
If the value is an Object it must be afunction object. The function's [[Call]] internal method (Table 7) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]
Boolean
Iftrue, the property is to be enumerated by a for-in enumeration (see14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]
Boolean
Iffalse, attempts to delete the property, change the property to be adata property, or change its attributes will fail.
If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined inTable 5is used.
Table 5: Default Attribute Values
Attribute Name
Default Value
[[Value]]
undefined
[[Get]]
undefined
[[Set]]
undefined
[[Writable]]
false
[[Enumerable]]
false
[[Configurable]]
false
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, aTypeErrorexception is thrown.
Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of anyECMAScript language typeor of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the valueundefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.
All objects have an internal slot named [[PrivateElements]], which is aListof PrivateElements. ThisListrepresents the values of the private fields, methods, and accessors for the object. Initially, it is an emptyList.
Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].
Table 6summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.
An ordinary object is an object that satisfies all of the following criteria:
For the internal methods listed inTable 6, the object uses those defined in10.1.
If the object has a [[Call]] internal method, it uses the one defined in10.2.1.
If the object has a [[Construct]] internal method, it uses the one defined in10.2.2.
An exotic object is an object that is not anordinary object.
This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind ofexotic object(such as anArray exotic objector abound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind ofexotic object.
The “Signature” column ofTable 6and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause6augmented by the following additional names. “any” means the value may be anyECMAScript language type.
In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.
Determine the object that provides inherited properties for this object. Anullvalue indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited properties. Passingnullindicates that there are no inherited properties. Returnstrueindicating that the operation was completed successfully orfalseindicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returnstrueif the operation was successful orfalseif the operation was unsuccessful.
Return aProperty Descriptorfor the own property of this object whose key is propertyKey, orundefinedif no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Returntrueif that property was successfully created/updated orfalseif the property could not be created or updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as thethisvalue when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) → Boolean
Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as thethisvalue when evaluating the code. Returnstrueif the property value was set orfalseif it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this object. Returnfalseif the property was not deleted and is still present. Returntrueif the property was deleted or is not present.
Return aListwhose elements are all of the own property keys for the object.
Table 7summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, everyconstructormust be afunction object. Therefore, aconstructormay also be referred to as a constructorfunction or constructorfunction object.
Table 7: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are athisvalue and aListwhose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
Creates an object. Invoked via the new operator or a super call. The first argument to the internal method is aListwhose elements are the arguments of theconstructorinvocation or the super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. Afunction objectis not necessarily aconstructorand such non-constructorfunction objects do not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause10. If any specified use of an internal method of anexotic objectis not supported by an implementation, that usage must throw aTypeErrorexception when attempted.
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to returnfalsefrom its [[IsExtensible]] internal method, ortruefrom its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a non-extensible target.
All references to SameValue are according to the definition of theSameValuealgorithm.
Return value:
The value returned by any internal method must be aCompletion Recordwith either:
[[Type]] =normal, [[Target]] =empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
An internal method must not return a completion with [[Type]] =continue,break, orreturn.
[[GetPrototypeOf]] ( )
The normal return type is either Object or Null.
If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return theSameValueas V.
Note 2
An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the valuenull). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use theordinary objectdefinition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must returnfalse, unless V is theSameValueas the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returnsfalse, all future calls to [[IsExtensible]] on the target must returnfalse.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returnstrue, all future calls to [[IsExtensible]] on the target must returnfalseand the target is now considered non-extensible.
If P is described as a non-configurable, non-writable owndata property, all future calls to [[GetOwnProperty]] ( P ) must returnProperty Descriptorwhose [[Value]] isSameValueas P's [[Value]] attribute.
If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must betrue.
If the [[Writable]] attribute may change fromfalsetotrue, then the [[Configurable]] attribute must betrue.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must returnundefined).
Note 3
As a consequence of the third invariant, if a property is described as adata propertyand it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must betrueeven if no mechanism to change the value is exposed via the other essential internal methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must returnfalseif P has previously been observed as a non-configurable own property of the target, unless either:
All attributes of Desc are theSameValueas P's attributes.
[[DefineOwnProperty]] (P, Desc) must returnfalseif target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data oraccessor propertyof the target, [[HasProperty]] must returntrue.
If P was previously observed as a non-configurable, non-writable owndata propertyof the target with value V, then [[Get]] must return theSameValueas V.
If P was previously observed as a non-configurable ownaccessor propertyof the target whose [[Get]] attribute isundefined, the [[Get]] operation must returnundefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable owndata propertyof the target, then [[Set]] must returnfalseunless V is theSameValueas P's [[Value]] attribute.
If P was previously observed as a non-configurable ownaccessor propertyof the target whose [[Set]] attribute isundefined, the [[Set]] operation must returnfalse.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data oraccessor propertyof the target, [[Delete]] must returnfalse.
The returnedListmust not contain any duplicate entries.
The Type of each element of the returnedListis either String or Symbol.
The returnedListmust contain at least the keys of all non-configurable own properties that have previously been observed.
If the target is non-extensible, the returnedListmust contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].
The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually haverealm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one perrealm.
Within this specification a reference such as %name% means the intrinsic object, associated with the currentrealm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the currentrealmand its intrinsics is described in9.4. The well-known intrinsics are listed inTable 8.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference,List,Completion,Property Descriptor,Environment Record,Abstract Closure, andData Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.
6.2.1 The List and Record Specification Types
The List type is used to explain the evaluation of argument lists (see13.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).
The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]:false, [[Field3]]:empty} defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]:false, [[Configurable]]:true}.
6.2.2 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in thememory model. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.
The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, aRb is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value R that satisfies the following.
For all a, b, and c in R's domain:
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 1
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value R that satisfies the following.
For all a, b, and c in R's domain:
a is identical to b or aRb or bRa, and
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 2
The three properties above are called totality, irreflexivity, and transitivity, respectively.
6.2.3 The Completion Record Specification Type
The Completion type is aRecordused to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.
Values of the Completion type areRecordvalues whose fields are defined byTable 9. Such values are referred to as Completion Records.
The following shorthand terms are sometimes used to refer to completions.
normal completion refers to any completion with a [[Type]] value ofnormal.
break completion refers to any completion with a [[Type]] value ofbreak.
continue completion refers to any completion with a [[Type]] value ofcontinue.
return completion refers to any completion with a [[Type]] value ofreturn.
throw completion refers to any completion with a [[Type]] value ofthrow.
abrupt completion refers to any completion with a [[Type]] value other thannormal.
Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of completion is considered an editorial error.
Implementation-definedcallable objects must return either a normal completion or a throw completion.
Set the code evaluation state of asyncContext such that when evaluation is resumed with aCompletioncompletion, the following steps of the algorithm that invokedAwaitwill be performed, with completion available.
Return.
NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of asyncContext.
where all aliases in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.
Note
Await can be combined with the ? and ! prefixes, so that for example
The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the superkeywordand other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.
A Reference Record is a resolved name or property binding; its fields are defined byTable 10.
If notempty, theReference Recordrepresents a property binding that was expressed using the superkeyword; it is called a Super Reference Record and its [[Base]] value will never be anEnvironment Record. In that case, the [[ThisValue]] field holds thethisvalue at the time theReference Recordwas created.
The followingabstract operationsare used in this specification to operate upon Reference Records:
6.2.4.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V. It performs the following steps when called:
The object that may be created in step4.ais not accessible outside of the above abstract operation and theordinary object[[Get]] internal method. An implementation might choose to avoid the actual creation of the object.
6.2.4.6 PutValue ( V, W )
The abstract operation PutValue takes arguments V and W. It performs the following steps when called:
The object that may be created in step5.ais not accessible outside of the above abstract operation and theordinary object[[Set]] internal method. An implementation might choose to avoid the actual creation of that object.
6.2.4.7 GetThisValue ( V )
The abstract operation GetThisValue takes argument V. It performs the following steps when called:
The abstract operation MakePrivateReference takes arguments baseValue (anECMAScript language value) and privateIdentifier (a String). It performs the following steps when called:
The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field's name is an attribute name and its value is a corresponding attribute value as specified in6.1.7.1. In addition, any field may be present or absent. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in eitherTable 3orTable 4.
The followingabstract operationsare used in this specification to operate upon Property Descriptor values:
6.2.5.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:
If Desc isundefined, returnfalse.
If both Desc.[[Get]] and Desc.[[Set]] are absent, returnfalse.
Returntrue.
6.2.5.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:
If Desc isundefined, returnfalse.
If both Desc.[[Value]] and Desc.[[Writable]] are absent, returnfalse.
Returntrue.
6.2.5.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:
If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return Desc.
6.2.6 The Environment Record Specification Type
TheEnvironment Recordtype is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in9.1.
6.2.7 The Abstract Closure Specification Type
The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Likeabstract operations, invocations perform the algorithm steps described by the Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.
The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is anintegervalue in the range 0 through 255, inclusive. A Data Block value is created with a fixed number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-originedinteger-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3rd byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.
The semantics of Shared Data Blocks is defined using Shared Data Block events by thememory model.Abstract operationsbelow introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of thememory model. The events form acandidate execution, on which thememory modelacts as a filter. Please consult thememory modelfor full semantics.
Shared Data Block events are modeled by Records, defined in thememory model.
The followingabstract operationsare used in this specification to operate upon Data Block values:
6.2.8.1 CreateByteDataBlock ( size )
The abstract operation CreateByteDataBlock takes argument size (aninteger). It performs the following steps when called:
The abstract operation CopyDataBlockBytes takes arguments toBlock, toIndex (a non-negativeinteger), fromBlock, fromIndex (a non-negativeinteger), and count (a non-negativeinteger). It performs the following steps when called:
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
Let bytes be aListwhose sole element is a nondeterministically chosenbyte value.
NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
The PrivateElement type is aRecordused in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.
Values of the PrivateElement type areRecordvalues whose fields are defined byTable 11. Such values are referred to as PrivateElements.
6.2.10 The ClassFieldDefinition Record Specification Type
The ClassFieldDefinition type is aRecordused in the specification of class fields.
Values of the ClassFieldDefinition type areRecordvalues whose fields are defined byTable 12. Such values are referred to as ClassFieldDefinition Records.
The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object withPrivateFieldAddorPrivateMethodOrAccessorAdd, and then read or written usingPrivateGetandPrivateSet.
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specializedabstract operationsare defined throughout this specification.
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversionabstract operations. The conversionabstract operationsare polymorphic; they can accept a value of anyECMAScript language type. But no other specification types are used with these operations.
The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.
7.1.1 ToPrimitive ( input [ , preferredType ] )
The abstract operation ToPrimitive takes argument input and optional argument preferredType. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:
When ToPrimitive is called with no hint, then it generally behaves as if the hint werenumber. However, objects may over-ride this behaviour by defining a@@toPrimitivemethod. Of the objects defined in this specification only Date objects (see21.4.4.45) and Symbol objects (see20.4.3.5) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint werestring.
7.1.1.1 OrdinaryToPrimitive ( O, hint )
The abstract operation OrdinaryToPrimitive takes arguments O and hint. It performs the following steps when called:
If argument is+0𝔽,-0𝔽, orNaN, returnfalse; otherwise returntrue.
String
If argument is the empty String (its length is 0), returnfalse; otherwise returntrue.
Symbol
Returntrue.
BigInt
If argument is0ℤ, returnfalse; otherwise returntrue.
Object
Returntrue.Note
An alternate algorithm related to the [[IsHTMLDDA]] internal slot is mandated in sectionB.3.6.1.
7.1.3 ToNumeric ( value )
The abstract operation ToNumeric takes argument value. It returns value converted to a Number or a BigInt. It performs the following steps when called:
The abstract operation RoundMVResult takes argument n (amathematical value). It converts n to a Number in animplementation-definedmanner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "themathematical valuedenoted by" a representation of amathematical valueis the inverse of "the decimal representation of" amathematical value. It performs the following steps when called:
If the decimal representation of n has 20 or fewer significant digits, return𝔽(n).
Let option1 be themathematical valuedenoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
Let option2 be themathematical valuedenoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
The abstract operation ToIntegerOrInfinity takes argument argument. It converts argument to aninteger, +∞, or -∞. It performs the following steps when called:
The abstract operation ToInt32 takes argument argument. It converts argument to one of 232integral Numbervalues in the range𝔽(-231) through𝔽(231 - 1), inclusive. It performs the following steps when called:
If int32bit ≥ 231, return𝔽(int32bit - 232); otherwise return𝔽(int32bit).
Note
Given the above definition of ToInt32:
The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that+∞𝔽 and-∞𝔽 are mapped to+0𝔽.)
ToInt32 maps-0𝔽 to+0𝔽.
7.1.7 ToUint32 ( argument )
The abstract operation ToUint32 takes argument argument. It converts argument to one of 232integral Numbervalues in the range+0𝔽 through𝔽(232 - 1), inclusive. It performs the following steps when called:
Step5is the only difference between ToUint32 andToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that+∞𝔽 and-∞𝔽 are mapped to+0𝔽.)
ToUint32 maps-0𝔽 to+0𝔽.
7.1.8 ToInt16 ( argument )
The abstract operation ToInt16 takes argument argument. It converts argument to one of 216integral Numbervalues in the range𝔽(-215) through𝔽(215 - 1), inclusive. It performs the following steps when called:
If int16bit ≥ 215, return𝔽(int16bit - 216); otherwise return𝔽(int16bit).
7.1.9 ToUint16 ( argument )
The abstract operation ToUint16 takes argument argument. It converts argument to one of 216integral Numbervalues in the range+0𝔽 through𝔽(216 - 1), inclusive. It performs the following steps when called:
The substitution of 216 for 232 in step4is the only difference betweenToUint32and ToUint16.
ToUint16 maps-0𝔽 to+0𝔽.
7.1.10 ToInt8 ( argument )
The abstract operation ToInt8 takes argument argument. It converts argument to one of 28integral Numbervalues in the range-128𝔽 through127𝔽, inclusive. It performs the following steps when called:
If int8bit ≥ 27, return𝔽(int8bit - 28); otherwise return𝔽(int8bit).
7.1.11 ToUint8 ( argument )
The abstract operation ToUint8 takes argument argument. It converts argument to one of 28integral Numbervalues in the range+0𝔽 through255𝔽, inclusive. It performs the following steps when called:
The abstract operation ToUint8Clamp takes argument argument. It converts argument to one of 28integral Numbervalues in the range+0𝔽 through255𝔽, inclusive. It performs the following steps when called:
Unlike the other ECMAScriptintegerconversion abstract operation, ToUint8Clamp rounds rather than truncates non-integral values and does not convert+∞𝔽 to+0𝔽. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:
If the MV isNaN, returnNaN, otherwise return the BigInt which exactly corresponds to the MV, rather than rounding to a Number.
7.1.15 ToBigInt64 ( argument )
The abstract operation ToBigInt64 takes argument argument. It converts argument to one of 264 BigInt values in the rangeℤ(-263) throughℤ(263-1), inclusive. It performs the following steps when called:
If int64bit ≥ 263, returnℤ(int64bit - 264); otherwise returnℤ(int64bit).
7.1.16 ToBigUint64 ( argument )
The abstract operation ToBigUint64 takes argument argument. It converts argument to one of 264 BigInt values in the range0ℤ through the BigInt value forℤ(264-1), inclusive. It performs the following steps when called:
Return a new Boolean object whose [[BooleanData]] internal slot is set to argument. See20.3for a description of Boolean objects.
Number
Return a new Number object whose [[NumberData]] internal slot is set to argument. See21.1for a description of Number objects.
String
Return a new String object whose [[StringData]] internal slot is set to argument. See22.1for a description of String objects.
Symbol
Return a new Symbol object whose [[SymbolData]] internal slot is set to argument. See20.4for a description of Symbol objects.
BigInt
Return a new BigInt object whose [[BigIntData]] internal slot is set to argument. See21.2for a description of BigInt objects.
Object
Return argument.
7.1.19 ToPropertyKey ( argument )
The abstract operation ToPropertyKey takes argument argument. It converts argument to a value that can be used as a property key. It performs the following steps when called:
The abstract operation ToLength takes argument argument. It converts argument to anintegral Numbersuitable for use as the length of anarray-like object. It performs the following steps when called:
The abstract operation CanonicalNumericIndexString takes argument argument. It returns argument converted to aNumber valueif it is a String representation of a Number that would be produced byToString, or the string"-0". Otherwise, it returnsundefined. It performs the following steps when called:
A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not returnundefined.
7.1.22 ToIndex ( value )
The abstract operation ToIndex takes argument value. It returns value argument converted to a non-negativeintegerif it is a validinteger indexvalue. It performs the following steps when called:
The abstract operation RequireObjectCoercible takes argument argument. It throws an error if argument is a value that cannot be converted to an Object usingToObject. It is defined byTable 18:
The abstract operation IsCallable takes argument argument (anECMAScript language value). It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:
If argument has a [[Call]] internal method, returntrue.
Returnfalse.
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (anECMAScript language value). It determines if argument is afunction objectwith a [[Construct]] internal method. It performs the following steps when called:
If argument has a [[Construct]] internal method, returntrue.
Returnfalse.
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether additional properties can be added to O. It performs the following steps when called:
The abstract operation IsIntegralNumber takes argument argument. It determines if argument is a finiteintegral Numbervalue. It performs the following steps when called:
The abstract operation IsPropertyKey takes argument argument (anECMAScript language value). It determines if argument is a value that may be used as a property key. It performs the following steps when called:
If matcher is notundefined, return ! ToBoolean(matcher).
If argument has a [[RegExpMatcher]] internal slot, returntrue.
Returnfalse.
7.2.9 IsStringPrefix ( p, q )
The abstract operation IsStringPrefix takes arguments p (a String) and q (a String). It determines if p is a prefix of q. It performs the following steps when called:
If q can be thestring-concatenationof p and some other String r, returntrue. Otherwise, returnfalse.
Note
Any String is a prefix of itself, because r may be the empty String.
7.2.10 SameValue ( x, y )
The abstract operation SameValue takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:
This algorithm differs from theIsStrictlyEqualAlgorithm in its treatment of signed zeroes and NaNs.
7.2.11 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:
SameValueZero differs fromSameValueonly in its treatment of+0𝔽 and-0𝔽.
7.2.12 SameValueNonNumeric ( x, y )
The abstract operation SameValueNonNumeric takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:
If x and y are both the same Symbol value, returntrue; otherwise, returnfalse.
If x and y are the same Object value, returntrue. Otherwise, returnfalse.
7.2.13 IsLessThan ( x, y, LeftFirst )
The abstract operation IsLessThan takes arguments x (anECMAScript language value), y (anECMAScript language value), and LeftFirst (a Boolean). It provides the semantics for the comparison x < y, returningtrue,false, orundefined(which indicates that at least one operand isNaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst istrue, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst isfalse, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:
Let k be the smallest non-negativeintegersuch that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
Let m be theintegerthat is the numeric value of the code unit at index k within px.
Let n be theintegerthat is the numeric value of the code unit at index k within py.
Step3differs from step2.cin the algorithm that handles the addition operator + (13.15.3) by using the logical-and operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.
7.2.14 IsLooselyEqual ( x, y )
The abstract operation IsLooselyEqual takes arguments x (anECMAScript language value) and y (anECMAScript language value). It provides the semantics for the comparison x == y, returningtrueorfalse. It performs the following steps when called:
The abstract operation IsStrictlyEqual takes arguments x (anECMAScript language value) and y (anECMAScript language value). It provides the semantics for the comparison x === y, returningtrueorfalse. It performs the following steps when called:
This algorithm differs from theSameValueAlgorithm in its treatment of signed zeroes and NaNs.
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:
Assert: internalSlotsList is aListof internal slot names.
Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the defaultordinary objectdefinitions specified in10.1.
Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] totrue.
Return obj.
Note
Within this specification, exotic objects are created inabstract operationssuch asArrayCreateandBoundFunctionCreateby first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulateexotic objectcreation, the object's essential internal methods are never modified outside those operations.
7.3.2 Get ( O, P )
The abstract operation Get takes arguments O (an Object) and P (a property key). It is used to retrieve the value of a specific property of an object. It performs the following steps when called:
The abstract operation GetV takes arguments V (anECMAScript language value) and P (a property key). It is used to retrieve the value of a specific property of anECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:
The abstract operation Set takes arguments O (an Object), P (a property key), V (anECMAScript language value), and Throw (a Boolean). It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:
If success isfalseand Throw istrue, throw aTypeErrorexception.
Return success.
7.3.5 CreateDataProperty ( O, P, V )
The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalse.
7.3.6 CreateMethodProperty ( O, P, V )
The abstract operation CreateMethodProperty takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalse.
7.3.7 CreateDataPropertyOrThrow ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It throws aTypeErrorexception if the requested property update cannot be performed. It performs the following steps when called:
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalsecausing this operation to throw aTypeErrorexception.
7.3.8 DefinePropertyOrThrow ( O, P, desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (aProperty Descriptor). It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw aTypeErrorexception if the requested property update cannot be performed. It performs the following steps when called:
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success isfalse, throw aTypeErrorexception.
Return success.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key). It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:
The abstract operation GetMethod takes arguments V (anECMAScript language value) and P (a property key). It is used to get the value of a specific property of anECMAScript language valuewhen the value of the property is expected to be a function. It performs the following steps when called:
The abstract operation HasProperty takes arguments O (an Object) and P (a property key). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. It performs the following steps when called:
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:
The abstract operation Call takes arguments F (anECMAScript language value) and V (anECMAScript language value) and optional argument argumentsList (aListof ECMAScript language values). It is used to call the [[Call]] internal method of afunction object. F is thefunction object, V is anECMAScript language valuethat is thethisvalue of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new emptyListis used as its value. It performs the following steps when called:
If argumentsList is not present, set argumentsList to a new emptyList.
The abstract operation Construct takes argument F (afunction object) and optional arguments argumentsList and newTarget. It is used to call the [[Construct]] internal method of afunction object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new emptyListis used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:
If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new emptyList.
If newTarget is not present, this operation is equivalent to: new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O and level. It is used to fix the set of own properties of an object. It performs the following steps when called:
The abstract operation TestIntegrityLevel takes arguments O and level. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:
The abstract operation CreateArrayFromList takes argument elements (aList). It is used to create an Array object whose elements are provided by elements. It performs the following steps when called:
Assert: elements is aListwhose elements are all ECMAScript language values.
The abstract operation LengthOfArrayLike takes argument obj. It returns the value of the"length"property of an array-like object (as a non-negativeinteger). It performs the following steps when called:
The abstract operation CreateListFromArrayLike takes argument obj and optional argument elementTypes (aListof names of ECMAScript Language Types). It is used to create aListvalue whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of theListthat is created. It performs the following steps when called:
If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
IfType(obj) is not Object, throw aTypeErrorexception.
IfType(next) is not an element of elementTypes, throw aTypeErrorexception.
Append next as the last element of list.
Set index to index + 1.
Return list.
7.3.20 Invoke ( V, P [ , argumentsList ] )
The abstract operation Invoke takes arguments V (anECMAScript language value) and P (a property key) and optional argument argumentsList (aListof ECMAScript language values). It is used to call a method property of anECMAScript language value. V serves as both the lookup point for the property and thethisvalue of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new emptyListis used as its value. It performs the following steps when called:
The abstract operation OrdinaryHasInstance takes arguments C (anECMAScript language value) and O. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:
7.3.22 SpeciesConstructor ( O, defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (aconstructor). It is used to retrieve theconstructorthat should be used to create new objects that are derived from O. defaultConstructor is theconstructorto use if aconstructor@@speciesproperty cannot be found starting from O. It performs the following steps when called:
The abstract operation EnumerableOwnPropertyNames takes arguments O (an Object) and kind (one ofkey,value, orkey+value). It performs the following steps when called:
The abstract operation PrivateFieldAdd takes arguments P (aPrivate Name), O (an Object), and value (anECMAScript language value). It performs the following steps when called:
AppendPrivateElement{ [[Key]]: P, [[Kind]]:field, [[Value]]: value } to O.[[PrivateElements]].
7.3.28 PrivateMethodOrAccessorAdd ( method, O )
The abstract operation PrivateMethodOrAccessorAdd takes arguments method (aPrivateElement) and O (an Object). It performs the following steps when called:
Assert: method.[[Kind]] is eithermethodoraccessor.
The abstract operation PrivateSet takes arguments P (aPrivate Name), O (an Object), and value (anECMAScript language value). It performs the following steps when called:
The abstract operation DefineField takes arguments receiver (an Object) and fieldRecord (aClassFieldDefinition Record). It performs the following steps when called:
7.3.32 InitializeInstanceElements ( O, constructor )
The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScriptfunction object). It performs the following steps when called:
Let methods be the value of constructor.[[PrivateMethods]].
The abstract operation IteratorStep takes argument iteratorRecord. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns eitherfalseindicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:
The abstract operation IteratorClose takes arguments iteratorRecord and completion. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
Assert:Type(iteratorRecord.[[Iterator]]) is Object.
The abstract operation AsyncIteratorClose takes arguments iteratorRecord and completion. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
Assert:Type(iteratorRecord.[[Iterator]]) is Object.
The abstract operation CreateIterResultObject takes arguments value and done. It creates an object that supports the IteratorResult interface. It performs the following steps when called:
The abstract operation CreateListIteratorRecord takes argument list. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:
Let closure be a newAbstract Closurewith no parameters that captures list and performs the following steps when called:
It is not necessary to treat export defaultAssignmentExpressionas a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.
The abstract operation IsAnonymousFunctionDefinition takes argument expr (aParse NodeforAssignmentExpressionor aParse NodeforInitializer). It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:
undefinedis passed for environment to indicate that aPutValueoperation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
Whenundefinedis passed for environment it indicates that aPutValueoperation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
Environment Record is a specification type used to define the association ofIdentifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as aFunctionDeclaration, aBlockStatement, or aCatchclause of aTryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is eithernullor a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if aFunctionDeclarationcontains two nestedFunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.
Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
A function Environment Record corresponds to the invocation of an ECMAScriptfunction object, and contains bindings for the top-level declarations within that function. It may establish a new this binding. It also captures the state necessary to support super method invocations.
An object Environment Record is used to define the effect of ECMAScript elements such asWithStatementthat associate identifier bindings with the properties of some object.
A global Environment Record is used forScriptglobal declarations. It does not have an outer environment; its [[OuterEnv]] isnull. It may be prepopulated with identifier bindings and it includes an associatedglobal objectwhose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to theglobal objectand the initial properties may be modified.
TheEnvironment Recordabstract class includes the abstract specification methods defined inTable 19. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
Table 19: Abstract Methods of Environment Records
Method
Purpose
HasBinding(N)
Determine if anEnvironment Recordhas a binding for the String value N. Returntrueif it does andfalseif it does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in anEnvironment Record. The String value N is the text of the bound name. If the Boolean argument D istruethe binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in anEnvironment Record. The String value N is the text of the bound name. If S istruethen attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in anEnvironment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of anyECMAScript language type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in anEnvironment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of anyECMAScript language type. S is a Boolean flag. If S istrueand the binding cannot be set throw aTypeErrorexception.
GetBindingValue(N, S)
Returns the value of an already existing binding from anEnvironment Record. The String value N is the text of the bound name. S is used to identify references originating instrict mode codeor that otherwise require strict mode reference semantics. If S istrueand the binding does not exist throw aReferenceErrorexception. If the binding exists but is uninitialized aReferenceErroris thrown, regardless of the value of S.
DeleteBinding(N)
Delete a binding from anEnvironment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and returntrue. If the binding exists but cannot be removed returnfalse. If the binding does not exist returntrue.
HasThisBinding()
Determine if anEnvironment Recordestablishes a this binding. Returntrueif it does andfalseif it does not.
HasSuperBinding()
Determine if anEnvironment Recordestablishes a super method binding. Returntrueif it does andfalseif it does not.
WithBaseObject()
If thisEnvironment Recordis associated with a with statement, return the with object. Otherwise, returnundefined.
9.1.1.1 Declarative Environment Records
Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of adeclarative Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:
If envRec has a binding for the name that is the value of N, returntrue.
Returnfalse.
9.1.1.1.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If D has the valuetrue, the new binding is marked as being subject to deletion. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D istrue, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
The CreateImmutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If S has the valuetrue, the new binding is marked as a strict binding. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S istrue, record that the newly created binding is a strict binding.
The InitializeBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist. It performs the following steps when called:
Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Recordthat the binding for N in envRec has been initialized.
The SetMutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, aTypeErroris thrown if S istrue. It performs the following steps when called:
An example of ECMAScript code that results in a missing binding at step1is:
functionf() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized aReferenceErroris thrown, regardless of the value of S. It performs the following steps when called:
If the binding for N in envRec is an uninitialized binding, throw aReferenceErrorexception.
Return the value currently bound to N in envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of adeclarative Environment RecordenvRec takes argument N (a String). It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:
Assert: envRec has a binding for the name that is the value of N.
If the binding for N in envRec cannot be deleted, returnfalse.
Remove the binding for N from envRec.
Returntrue.
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of adeclarative Environment RecordenvRec takes no arguments. It performs the following steps when called:
The WithBaseObject concrete method of adeclarative Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnundefined.
9.1.1.2 Object Environment Records
Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of anIdentifierNameare not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the valuefalse. Immutable bindings do not exist for object Environment Records.
Object Environment Records created for with statements (14.11) can provide their binding object as an implicitthisvalue for use in function calls. The capability is controlled by a Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed inTable 20.
Table 20: Additional Fields of Object Environment Records
Indicates whether thisEnvironment Recordis created for a with statement.
The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of anobject Environment RecordenvRec takes argument N (a String). It determines if its associated binding object has a property whose name is the value of the argument N. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let foundBinding be ? HasProperty(bindingObject, N).
If foundBinding isfalse, returnfalse.
If envRec.[[IsWithEnvironment]] isfalse, returntrue.
The CreateMutableBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates in anEnvironment Record's associated binding object a property whose name is the String value and initializes it to the valueundefined. If D has the valuetrue, the new property's [[Configurable]] attribute is set totrue; otherwise it is set tofalse. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Return ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor { [[Value]]:undefined, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]: D }).
Note
Normally envRec will not have a binding for N but if it does, the semantics ofDefinePropertyOrThrowmay result in an existing binding being replaced or shadowed or cause anabrupt completionto be returned.
9.1.1.2.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of anobject Environment Recordis never used within this specification.
9.1.1.2.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. It performs the following steps when called:
Return ? envRec.SetMutableBinding(N, V,false).
Note
In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in object Environment Records.
9.1.1.2.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to set the value of theEnvironment Record's associated binding object's property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let stillExists be ? HasProperty(bindingObject, N).
If stillExists isfalseand S istrue, throw aReferenceErrorexception.
The GetBindingValue concrete method of anobject Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its associated binding object's property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon S. It performs the following steps when called:
The DeleteBinding concrete method of anobject Environment RecordenvRec takes argument N (a String). It can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the valuetrue. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnfalse.
Note
Object Environment Records do not provide a this binding.
9.1.1.2.9 HasSuperBinding ( )
The HasSuperBinding concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnfalse.
Note
Object Environment Records do not provide a super binding.
9.1.1.2.10 WithBaseObject ( )
The WithBaseObject concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:
If envRec.[[IsWithEnvironment]] istrue, return envRec.[[BindingObject]].
Otherwise, returnundefined.
9.1.1.3 Function Environment Records
A function Environment Record is adeclarative Environment Recordthat is used to represent the top-level scope of a function and, if the function is not anArrowFunction, provides a this binding. If a function is not anArrowFunctionfunction and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed inTable 21.
Table 21: Additional Fields of Function Environment Records
Field Name
Value
Meaning
[[ThisValue]]
Any
This is thethisvalue used for this invocation of the function.
[[ThisBindingStatus]]
lexical|initialized|uninitialized
If the value islexical, this is anArrowFunctionand does not have a localthisvalue.
If thisEnvironment Recordwas created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise, its value isundefined.
Function Environment Records support all of thedeclarative Environment Recordmethods listed inTable 19and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed inTable 22:
Table 22: Additional Methods of Function Environment Records
Method
Purpose
BindThisValue(V)
Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()
Return the value of thisEnvironment Record's this binding. Throws aReferenceErrorif the this binding has not been initialized.
GetSuperBase()
Return the object that is the base for super property accesses bound in thisEnvironment Record. The valueundefinedindicates that super property accesses will produce runtime errors.
The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:
A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScriptScriptelements that are processed in a commonrealm. A global Environment Record provides the bindings for built-in globals (clause19), properties of theglobal object, and for all top-level declarations (8.1.9,8.1.11) that occur within aScript.
Determines if the argument is the name of aglobal objectproperty that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N)
Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D)
Used to create and initialize toundefineda global var binding in the [[ObjectRecord]] component of aglobal Environment Record. The binding will be a mutable binding. The correspondingglobal objectproperty will have attribute values appropriate for a var. The String value N is the bound name. If D istruethe binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global function binding in the [[ObjectRecord]] component of aglobal Environment Record. The binding will be a mutable binding. The correspondingglobal objectproperty will have attribute values appropriate for a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D istruethe binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.
The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.
9.1.1.4.1 HasBinding ( N )
The HasBinding concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) istrue, returntrue.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If D has the valuetrue, the new binding is marked as being subject to deletion. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) istrue, throw aTypeErrorexception.
Return DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If S has the valuetrue, the new binding is marked as a strict binding. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) istrue, throw aTypeErrorexception.
Return DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist. It performs the following steps when called:
The SetMutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, aTypeErroris thrown if S istrue. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) istrue, then
Return DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N, V, S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw aReferenceErrorexception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If DclRec.HasBinding(N) istrue, then
Return DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N, S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of aglobal Environment RecordenvRec takes argument N (a String). It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:
If N is an element of varNames, remove that element from the varNames.
Return status.
Returntrue.
9.1.1.4.8 HasThisBinding ( )
The HasThisBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returntrue.
Note
Global Environment Records always provide a this binding.
9.1.1.4.9 HasSuperBinding ( )
The HasSuperBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnfalse.
Note
Global Environment Records do not provide a super binding.
9.1.1.4.10 WithBaseObject ( )
The WithBaseObject concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnundefined.
9.1.1.4.11 GetThisBinding ( )
The GetThisBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:
Return envRec.[[GlobalThisValue]].
9.1.1.4.12 HasVarDeclaration ( N )
The HasVarDeclaration concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier has a binding in this record that was created using aVariableStatementor aFunctionDeclaration. It performs the following steps when called:
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains N, returntrue.
Returnfalse.
9.1.1.4.13 HasLexicalDeclaration ( N )
The HasLexicalDeclaration concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier has a binding in this record that was created using a lexical declaration such as aLexicalDeclarationor aClassDeclaration. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
Return DclRec.HasBinding(N).
9.1.1.4.14 HasRestrictedGlobalProperty ( N )
The HasRestrictedGlobalProperty concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is the name of a property of theglobal objectthat must not be shadowed by a global lexical binding. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp isundefined, returnfalse.
If existingProp.[[Configurable]] istrue, returnfalse.
Returntrue.
Note
Properties may exist upon aglobal objectthat were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of theglobal object. The global property"undefined"is an example of such a property.
9.1.1.4.15 CanDeclareGlobalVar ( N )
The CanDeclareGlobalVar concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existingglobal objectproperties are allowed. It performs the following steps when called:
The CanDeclareGlobalFunction concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp isundefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] istrue, returntrue.
IfIsDataDescriptor(existingProp) istrueand existingProp has attribute values { [[Writable]]:true, [[Enumerable]]:true}, returntrue.
Returnfalse.
9.1.1.4.17 CreateGlobalVarBinding ( N, D )
The CreateGlobalVarBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates and initializes a mutable binding in the associatedobject Environment Recordand records the bound name in the associated [[VarNames]]List. If a binding already exists, it is reused and assumed to be initialized. It performs the following steps when called:
9.1.1.4.18 CreateGlobalFunctionBinding ( N, V, D )
The CreateGlobalFunctionBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and D (a Boolean). It creates and initializes a mutable binding in the associatedobject Environment Recordand records the bound name in the associated [[VarNames]]List. If a binding already exists, it is replaced. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp isundefinedor existingProp.[[Configurable]] istrue, then
Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]: D }.
Else,
Let desc be the PropertyDescriptor { [[Value]]: V }.
Global function declarations are always represented as own properties of theglobal object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step7is equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.
9.1.1.5 Module Environment Records
A module Environment Record is adeclarative Environment Recordthat is used to represent the outer scope of an ECMAScriptModule. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in anotherEnvironment Record.
Module Environment Records support all of thedeclarative Environment Recordmethods listed inTable 19and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed inTable 25:
Table 25: Additional Methods of Module Environment Records
The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:
9.1.1.5.1 GetBindingValue ( N, S )
The GetBindingValue concrete method of amodule Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized aReferenceErroris thrown. It performs the following steps when called:
The HasThisBinding concrete method of amodule Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returntrue.
Note
Module Environment Records always provide a this binding.
9.1.1.5.4 GetThisBinding ( )
The GetThisBinding concrete method of amodule Environment RecordenvRec takes no arguments. It performs the following steps when called:
Returnundefined.
9.1.1.5.5 CreateImportBinding ( N, M, N2 )
The CreateImportBinding concrete method of amodule Environment RecordenvRec takes arguments N (a String), M (aModule Record), and N2 (a String). It creates a new initialized immutable indirect binding for the name N. A binding must not already exist in thisEnvironment Recordfor N. N2 is the name of a binding that exists in M'smodule Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
The abstract operation GetIdentifierReference takes arguments env (anEnvironment Recordornull), name (a String), and strict (a Boolean). It performs the following steps when called:
The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a Boolean), and E (anEnvironment Recordornull). It performs the following steps when called:
A PrivateEnvironment Record is a specification mechanism used to track Private Names based upon the lexical nesting structure ofClassDeclarations andClassExpressions in ECMAScript code. They are similar to, but distinct from, Environment Records. EachPrivateEnvironment Recordis associated with aClassDeclarationorClassExpression. Each time such a class is evaluated, a newPrivateEnvironment Recordis created to record the Private Names declared by that class.
The followingabstract operationsare used in this specification to operate upon PrivateEnvironment Records:
9.2.1.1 NewPrivateEnvironment ( outerPrivEnv )
The abstract operation NewPrivateEnvironment takes argument outerPrivEnv (aPrivateEnvironment Recordornull). It performs the following steps when called:
The abstract operation ResolvePrivateIdentifier takes arguments privEnv (aPrivateEnvironment Record) and identifier (a String). It performs the following steps when called:
Let names be privEnv.[[Names]].
If names contains aPrivate Namewhose [[Description]] is identifier, then
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, arealmconsists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.
Arealmis represented in this specification as a Realm Record with the fields specified inTable 27:
Template objects are canonicalized separately for eachrealmusing itsRealm Record's [[TemplateMap]]. Each [[Site]] value is aParse Nodethat is aTemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function.
Note
Once aParse Nodebecomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
[[HostDefined]]
Any, default value isundefined.
Field reserved for use by hosts that need to associate additional information with aRealm Record.
9.3.1 CreateRealm ( )
The abstract operation CreateRealm takes no arguments. It performs the following steps when called:
Set fields of intrinsics with the values listed inTable 8. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses19through28. All object property values are newly created object values. All values that are built-in function objects are created by performingCreateBuiltinFunction(steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function's name property, length is the initial value of the function's length property, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Let desc be the fully populated dataProperty Descriptorfor the property, containing the specified attributes for the property. For properties listed in19.2,19.3, or19.4the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context peragentthat is actually executing code. This is known as theagent's running execution context. All references to therunning execution contextin this specification denote therunning execution contextof thesurrounding agent.
The execution context stack is used to track execution contexts. Therunning execution contextis always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currentlyrunning execution contextto executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes therunning execution context.
An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed inTable 28.
Table 28: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with thisexecution context.
Evaluation of code by therunning execution contextmay be suspended at various points defined within this specification. Once therunning execution contexthas been suspended a different execution context may become therunning execution contextand commence evaluating its code. At some later time a suspended execution context may again become therunning execution contextand continue evaluating its code at the point where it had previously been suspended. Transition of therunning execution contextstatus among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of therunning execution context.
Identifies thePrivateEnvironment Recordthat holds Private Names created byClassElements in the nearest containing class.nullif there is no containing class.
The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.
Execution contexts representing the evaluation of generator objects have the additional state components listed inTable 30.
Table 30: Additional State Components for Generator Execution Contexts
In most situations only therunning execution context(the top of theexecution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of therunning execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
9.4.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments. It is used to determine the running script or module, based on therunning execution context. It performs the following steps when called:
If no suchexecution contextexists, returnnull. Otherwise, return ec's ScriptOrModule.
9.4.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional argument env (anEnvironment Record). It is used to determine the binding of name. env can be used to explicitly provide theEnvironment Recordthat is to be searched for the binding. It performs the following steps when called:
The result of ResolveBinding is always aReference Recordwhose [[ReferencedName]] field is name.
9.4.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments. It finds theEnvironment Recordthat currently supplies the binding of thekeywordthis. It performs the following steps when called:
The loop in step2will always terminate because the list of environments always ends with the global environment which has a this binding.
9.4.4 ResolveThisBinding ( )
The abstract operation ResolveThisBinding takes no arguments. It determines the binding of thekeywordthis using the LexicalEnvironment of therunning execution context. It performs the following steps when called:
The abstract operation GetNewTarget takes no arguments. It determines the NewTarget value using the LexicalEnvironment of therunning execution context. It performs the following steps when called:
The abstract operation GetGlobalObject takes no arguments. It returns theglobal objectused by the currentlyrunning execution context. It performs the following steps when called:
A Job is anAbstract Closurewith no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.
Jobs are scheduled for execution by ECMAScripthostenvironments. This specification describes thehost hookHostEnqueuePromiseJobto schedule one kind of job; hosts may define additionalabstract operationswhich schedule jobs. Such operations accept aJobAbstract Closureas the parameter and schedule it to be performed at some future time. Their implementations must conform to the following requirements:
Hostenvironments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.
At any particular time, scriptOrModule (aScript Record, aModule Record, ornull) is the active script or module if all of the following conditions are true:
The specific choice ofRealmis up to thehost environment. This initialexecution contextandRealmis only in use before any callback function is invoked. When a callback function related to aJob, like a Promise handler, is invoked, the invocation pushes its ownexecution contextandRealm.
Particular kinds of Jobs have additional conformance requirements.
The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses thehost-definedvalue to propagate the incumbent settings object for Promise callbacks.
JobCallback Records have the fields listed inTable 31.
ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reactionJob.
ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.
9.5.4 HostEnqueuePromiseJob ( job, realm )
Thehost-definedabstract operation HostEnqueuePromiseJob takes arguments job (aJobAbstract Closure) and realm (aRealm Recordornull). It schedules job to be performed at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in9.5as well as the following:
Let scriptOrModule beGetActiveScriptOrModule() at the time HostEnqueuePromiseJob is invoked. If realm is notnull, each time job is invoked the implementation must performimplementation-definedsteps such that scriptOrModule is theactive script or moduleat the time of job's invocation.
Jobs must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.
Note
The realm for Jobs returned byNewPromiseResolveThenableJobis usually the result of callingGetFunctionRealmon the thenfunction object. The realm for Jobs returned byNewPromiseReactionJobis usually the result of callingGetFunctionRealmon the handler if the handler is notundefined. If the handler isundefined, realm isnull. For both kinds of Jobs, whenGetFunctionRealmcompletes abnormally (i.e. called on a revoked Proxy), realm is the currentRealmat the time of theGetFunctionRealmcall. When the realm isnull, no user ECMAScript code will be evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses realm to check for the ability to run script and for the entry concept.
9.6 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments. It performs the following steps when called:
If thehostrequires that the this binding in realm's global scope return an object other than theglobal object, let thisValue be such an object created in ahost-definedmanner. Otherwise, let thisValue beundefined, indicating that realm's global this binding should be theglobal object.
Anagent'sexecuting threadexecutes a job on theagent's execution contexts independently of other agents, except that anexecuting threadmay be used as theexecuting threadby multiple agents, provided none of the agents sharing the thread have anAgent Recordwhose [[CanBlock]] property istrue.
Note 1
Some web browsers share a singleexecuting threadacross multiple unrelated tabs of a browser window, for example.
The default value computed for the isLittleEndian parameter when it is needed by the algorithmsGetValueFromBufferandSetValueInBuffer. The choice isimplementation-definedand should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
Initially a new emptyList, representing the list of objects to be kept alive until the end of the currentJob
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by anyagentin theagent clusterthey cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.
There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. SeeReadSharedMemoryandWriteSharedMemoryfor details.
Note 3
Anagentis a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.7.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments. It performs the following steps when called:
In some environments it may not be reasonable for a givenagentto suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.
9.8 Agent Clusters
An agent cluster is a maximal set of agents that can communicate by operating on shared memory.
Note 1
Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.
There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.
The agents in a cluster need not all be alive at some particular point in time. IfagentA creates anotheragentB, after which A terminates and B createsagentC, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.
All agents within a cluster must have the same value for the [[LittleEndian]] property in their respectiveAgentRecords.
Note 3
If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.
All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respectiveAgentRecords; similarly for the [[IsLockFree2]] property.
All agents within a cluster must have different values for the [[Signifier]] property in their respectiveAgentRecords.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) anagentwithout theagent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where anagentdeadlocks or starves because anotheragenthas been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.
The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.
An embedding may terminate anagentwithout any of theagent's cluster's other agents' prior knowledge or cooperation. If anagentis terminated not by programmatic action of its own or of anotheragentin the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify theagentthat was terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating anagentthat is running in-process with the other agents when per-agentresource accounting indicates that theagentis runaway.
Prior to any evaluation of any ECMAScript code by anyagentin a cluster, the [[CandidateExecution]] field of theAgent Recordfor all agents in the cluster is set to the initialcandidate execution. The initialcandidate executionis anempty candidate executionwhose [[EventsRecords]] field is aListcontaining, for eachagent, anAgent Events Recordwhose [[AgentSignifier]] field is thatagent's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.
An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.9 Forward Progress
For anagentto make forward progress is for it to perform an evaluation step according to this specification.
Anagentbecomes blocked when itsrunning execution contextwaits synchronously and indefinitely for an external event. Only agents whoseAgent Record's [[CanBlock]] property istruecan become blocked in this sense. An unblockedagentis one that is not blocked.
Implementations must ensure that:
every unblockedagentwith a dedicatedexecuting threadeventually makes forward progress
in a set of agents that share anexecuting thread, oneagenteventually makes forward progress
anagentdoes not cause anotheragentto become blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in thememory model, ensures that allSeqCstwrites eventually become observable to all agents.
9.10 Processing Model of WeakRef and FinalizationRegistry Objects
9.10.1 Objectives
This specification does not make any guarantees that any object will be garbage collected. Objects which are notlivemay be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour triggered by garbage collection.
The semantics ofWeakRefandFinalizationRegistryobjects is based on two operations which happen at particular points in time:
When WeakRef.prototype.deref is called, the referent (ifundefinedis not returned) is kept alive so that subsequent, synchronous accesses also return the object. This list is reset when synchronous work is done using theClearKeptObjectsabstract operation.
Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting thehost environmentscheduleCleanupFinalizationRegistryallows it to resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory usage.
9.10.2 Liveness
For some set of objects S, a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operationWeakRefDerefof aWeakRefwhose referent is an element of S always returnsundefined.
Note 1
WeakRef-obliviousness, together with liveness, capture two notions. One, that aWeakRefitself does not keep an object alive. Two, that cycles in liveness does not imply that an object is live. To be concrete, if determining obj's liveness depends on determining the liveness of anotherWeakRefreferent, obj2, obj2's liveness cannot assume obj's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects instead of individual objects to account for cycles. If it were defined on individual objects, then an object in a cycle will be considered live even though its Object value is only observed via WeakRefs of other objects in the cycle.
Note 3
Colloquially, we say that an individual object is live if every set of objects containing it is live.
At any point during evaluation, a set of objects S is considered live if either of the following conditions is met:
Any element in S is included in anyagent's [[KeptAlive]]List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the Object value of any object in S.
Note 4
The second condition above intends to capture the intuition that an object is live if its identity is observable via non-WeakRefmeans. An object's identity may be observed by observing a strict equality comparison between objects or observing the object being used as key in a Map.
Note 5
Presence of an object in a field, an internal slot, or a property does not imply that the object is live. For example if the object in question is never passed back to the program, then it cannot be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of aFinalizationRegistryCell record.
The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative approximations such as reachability. There is expected to be significant implementation leeway.
9.10.3 Execution
At any time, if a set of objects S is notlive, an ECMAScript implementation may perform the following steps atomically:
For each element obj of S, do
For eachWeakRefref such that ref.[[WeakRefTarget]] is obj, do
Set ref.[[WeakRefTarget]] toempty.
For eachFinalizationRegistryfg such that fg.[[Cells]] contains aRecordcell such that cell.[[WeakRefTarget]] is obj, do
For each WeakMap map such that map.[[WeakMapData]] contains aRecordr such that r.[[Key]] is obj, do
Set r.[[Key]] toempty.
Set r.[[Value]] toempty.
For each WeakSet set such that set.[[WeakSetData]] contains obj, do
Replace the element of set.[[WeakSetData]] whose value is obj with an element whose value isempty.
Note 1
Together with the definition of liveness, this clause prescribes legal optimizations that an implementation may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of aWeakRef, optimizations such as rematerialization that observably empty theWeakRefare prohibited.
Implementations are not obligated to empty WeakRefs for maximal sets of non-liveobjects.
If an implementation chooses a non-liveset S in which to empty WeakRefs, it must empty WeakRefs for all objects in S simultaneously. In other words, an implementation must not empty aWeakRefpointing to an object obj without emptying out other WeakRefs that, if not emptied, could result in an execution that observes the Object value of obj.
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be performed at some future time, if possible. It must also conform to the requirements in9.5.
9.11 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:
When the abstract operation AddToKeptObjects is called with a target object reference, it adds the target to a list that will point strongly at the target untilClearKeptObjectsis called.
The abstract operation CleanupFinalizationRegistry takes argument finalizationRegistry (aFinalizationRegistry). It performs the following steps when called:
Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
Let callback be finalizationRegistry.[[CleanupCallback]].
While finalizationRegistry.[[Cells]] contains aRecordcell such that cell.[[WeakRefTarget]] isempty, an implementation may perform the following steps:
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is eithernullor an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (and visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.
Everyordinary objecthas a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set tofalse, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] totrue.
Eachordinary objectinternal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked whenordinary objectinternal methods are applied to them.
10.1.1 [[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (anECMAScript language value). It performs the following steps when called:
If p.[[GetPrototypeOf]] is not theordinary objectinternal method defined in10.1.1, set done totrue.
Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Returntrue.
Note
The loop in step8guarantees that there will be no circularities in any prototype chain that only includes objects that use theordinary objectdefinitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3 [[IsExtensible]] ( )
The [[IsExtensible]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:
Set D.[[Get]] to the value of X's [[Get]] attribute.
Set D.[[Set]] to the value of X's [[Set]] attribute.
Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
Return D.
10.1.6 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of anordinary objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:
The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (aProperty Descriptor). It performs the following steps when called:
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (aProperty Descriptor), and Current (aProperty Descriptor). It performs the following steps when called:
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object orundefined), P (a property key), extensible (a Boolean), Desc (aProperty Descriptor), and current (aProperty Descriptor).
Note
Ifundefinedis passed as O, only validation is performed and no object updates are performed.
If O is notundefined, create an owndata propertynamed P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value.
If O is notundefined, create an ownaccessor propertynamed P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value.
Returntrue.
If every field in Desc is absent, returntrue.
If current.[[Configurable]] isfalse, then
If Desc.[[Configurable]] is present and its value istrue, returnfalse.
If Desc.[[Enumerable]] is present and ! SameValue(Desc.[[Enumerable]], current.[[Enumerable]]) isfalse, returnfalse.
If O is notundefined, convert the property named P of object O from adata propertyto anaccessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values.
Else,
If O is notundefined, convert the property named P of object O from anaccessor propertyto adata property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values.
The [[Get]] internal method of anordinary objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:
The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (anECMAScript language value). It performs the following steps when called:
The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:
For each own property key P of O such that P is anarray index, in ascending numeric index order, do
Add P as the last element of keys.
For each own property key P of O such thatType(P) is String and P is not anarray index, in ascending chronological order of property creation, do
Add P as the last element of keys.
For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
Add P as the last element of keys.
Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object ornull) and optional argument additionalInternalSlotsList (aListof names of internal slots). It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new emptyListis used. It performs the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, append each of its elements to internalSlotsList.
Although OrdinaryObjectCreate does little more than callMakeBasicObject, its use communicates the intention to create anordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invokeMakeBasicObjectdirectly.
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor and intrinsicDefaultProto and optional argument internalSlotsList (aListof names of internal slots). It creates anordinary objectwhose [[Prototype]] value is retrieved from aconstructor's"prototype"property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new emptyListis used. It performs the following steps when called:
Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor takes arguments constructor and intrinsicDefaultProto. It determines the [[Prototype]] value that should be used to create an object corresponding to a specificconstructor. The value is retrieved from theconstructor's"prototype"property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:
Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Set proto to realm's intrinsic object named intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from therealmof the constructor function rather than from therunning execution context.
10.1.15 RequireInternalSlot ( O, internalSlot )
The abstract operation RequireInternalSlot takes arguments O and internalSlot. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:
IfType(O) is not Object, throw aTypeErrorexception.
If O does not have an internalSlot internal slot, throw aTypeErrorexception.
10.2 ECMAScript Function Objects
ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScriptfunction objectis anordinary objectand has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScriptfunction objectmay be eitherstrict mode code(11.2.2) ornon-strict code. An ECMAScriptfunction objectwhose code isstrict mode codeis called a strict function. One whose code is notstrict mode codeis called a non-strict function.
In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed inTable 33.
Table 33: Internal Slots of ECMAScript Function Objects
ThePrivateEnvironment Recordfor Private Names that the function was closed over.nullif this function is not syntactically contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of the function.
The script or module in which the function was created.
[[ThisMode]]
lexical|strict|global
Defines how this references are interpreted within the formal parameters and code body of the function.lexicalmeans that this refers to thethisvalue of a lexically enclosing function.strictmeans that thethisvalue is used exactly as provided by an invocation of the function.globalmeans that athisvalue ofundefinedornullis interpreted as a reference to theglobal object, and any otherthisvalue is first passed toToObject.
If the function is created as the initializer of a class field, the name to use forNamedEvaluationof the field;emptyotherwise.
[[IsClassConstructor]]
Boolean
Indicates whether the function is a classconstructor. (Iftrue, invoking the function's [[Call]] will immediately throw aTypeErrorexception.)
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
10.2.1 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of an ECMAScriptfunction objectF takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:
When calleeContext is removed from theexecution context stackin step8it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.
10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (afunction object) and newTarget (anECMAScript language value). It performs the following steps when called:
Even though field initializers constitute a function boundary, callingFunctionDeclarationInstantiationdoes not have any observable effect and so is omitted.
The abstract operation OrdinaryCallEvaluateBody takes arguments F (afunction object) and argumentsList (aList). It performs the following steps when called:
Return the result ofEvaluateBodyof the parsed code that is F.[[ECMAScriptCode]] passing F and argumentsList as the arguments.
10.2.2 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of an ECMAScriptfunction objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (aParse Node), Body (aParse Node), thisMode (eitherlexical-thisornon-lexical-this), Scope (anEnvironment Record), and PrivateScope (aPrivateEnvironment Recordornull). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:
The abstract operation AddRestrictedFunctionProperties takes arguments F (afunction object) and realm (aRealm Record). It performs the following steps when called:
The %ThrowTypeError% intrinsic is an anonymous built-infunction objectthat is defined once for eachrealm. When %ThrowTypeError% is called it performs the following steps:
Throw aTypeErrorexception.
The value of the [[Extensible]] internal slot of a %ThrowTypeError% function isfalse.
The"length"property of a %ThrowTypeError% function has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
The"name"property of a %ThrowTypeError% function has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
The abstract operation MakeConstructor takes argument F (afunction object) and optional arguments writablePrototype (a Boolean) and prototype (an Object). It converts F into aconstructor. It performs the following steps when called:
The abstract operation DefineMethodProperty takes arguments key (a property key orPrivate Name), homeObject (an Object), closure (afunction object), and enumerable (a Boolean). It performs the following steps when called:
The abstract operation SetFunctionName takes arguments F (afunction object) and name (a property key orPrivate Name) and optional argument prefix (a String). It adds a"name"property to F. It performs the following steps when called:
Assert: F is an extensible object that does not have a"name"own property.
The abstract operation SetFunctionLength takes arguments F (afunction object) and length (a non-negativeintegeror +∞). It adds a"length"property to F. It performs the following steps when called:
Assert: F is an extensible object that does not have a"length"own property.
The abstract operation FunctionDeclarationInstantiation takes arguments func (afunction object) and argumentsList. func is thefunction objectfor which theexecution contextis being established.
Note 1
When anexecution contextis established for evaluating an ECMAScript function a newfunction Environment Recordis created and bindings for each formal parameter are instantiated in thatEnvironment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the sameEnvironment Recordas the parameters. If default value parameter initializers exist, a secondEnvironment Recordis created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.
NOTE: If there are multiple function declarations for the same name, the last declaration is used.
Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded betrue.
If func.[[ThisMode]] islexical, then
NOTE: Arrow functions never have an arguments objects.
Set argumentsObjectNeeded tofalse.
Else if"arguments"is an element of parameterNames, then
Set argumentsObjectNeeded tofalse.
Else if hasParameterExpressions isfalse, then
If"arguments"is an element of functionNames or if"arguments"is an element of lexicalNames, then
Set argumentsObjectNeeded tofalse.
If strict istrueor if hasParameterExpressions isfalse, then
NOTE: Only a singleEnvironment Recordis needed for the parameters and top-level vars.
Let env be the LexicalEnvironment of calleeContext.
Else,
NOTE: A separateEnvironment Recordis needed to ensure that bindings created bydirect evalcalls in the formal parameter list are outside the environment where parameters are declared.
Let calleeEnv be the LexicalEnvironment of calleeContext.
Assert: The VariableEnvironment of calleeContext is calleeEnv.
Set the LexicalEnvironment of calleeContext to env.
For each String paramName of parameterNames, do
Let alreadyDeclared be env.HasBinding(paramName).
NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
NOTE: Only a singleEnvironment Recordis needed for the parameters and top-level vars.
Let instantiatedVarNames be a copy of theListparameterBindings.
For each element n of varNames, do
If n is not an element of instantiatedVarNames, then
Append n to instantiatedVarNames.
Perform ! env.CreateMutableBinding(n,false).
Call env.InitializeBinding(n,undefined).
Let varEnv be env.
Else,
NOTE: A separateEnvironment Recordis needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
NOTE: Non-strict functions use a separateEnvironment Recordfor top-level lexical declarations so that adirect evalcan determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strictdirect evalalways places all declarations into a newEnvironment Record.
Else, let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
B.3.2provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.
10.3 Built-in Function Objects
The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (10.2) whose behaviour is provided using ECMAScript code or as implementation provided function exotic objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.
If a built-infunction objectis implemented as an ECMAScriptfunction object, it must have all the internal slots described in10.2([[Prototype]], [[Extensible]], and the slots listed inTable 33), plus [[InitialName]]. The value of the [[InitialName]] internal slot is a String value that is the initial name of the function. It is used by20.2.3.5.
If a built-infunction objectis implemented as anexotic object, it must have theordinary objectbehaviour specified in10.1. All such function exotic objects have [[Prototype]], [[Extensible]], [[Realm]], and [[InitialName]] internal slots, with the same meanings as above.
Unless otherwise specified every built-infunction objecthas the%Function.prototype%object as the initial value of its [[Prototype]] internal slot.
The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides thethisvalue, the [[Call]] argumentsList provides the named parameters, and the NewTarget value isundefined. When invoked with [[Construct]], thethisvalue is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScriptfunction objectthen this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-inconstructorhas any [[Call]] behaviour other than throwing aTypeErrorexception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[IsClassConstructor]] internal slot to have the valuetrue.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-inconstructoris called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-inconstructor's named parameters.
Built-in functions that are not constructors do not have a"prototype"property unless otherwise specified in the description of a particular function.
If a built-infunction objectis not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:
10.3.1 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of a built-infunction objectF takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:
Let result be theCompletion Recordthat is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is thethisvalue, argumentsList provides the named parameters, and the NewTarget value isundefined.
When calleeContext is removed from theexecution context stackit must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.
10.3.2 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of a built-infunction objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). The steps performed are the same as [[Call]] (see10.3.1) except that step10is replaced by:
Let result be theCompletion Recordthat is the result of evaluating F in a manner that conforms to the specification of F. Thethisvalue is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
The abstract operation CreateBuiltinFunction takes arguments behaviour, length (a non-negativeintegeror +∞), name (a property key), and additionalInternalSlotsList (aListof names of internal slots) and optional arguments realm (aRealm Record), prototype (an Object ornull), and prefix (a String). additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-infunction object. It performs the following steps when called:
Assert: behaviour is either anAbstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification.
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
Let internalSlotsList be aListcontaining the names of all the internal slots that10.3requires for the built-infunction objectthat is about to be created.
Append to internalSlotsList the elements of additionalInternalSlotsList.
Let func be a new built-infunction objectthat, when called, performs the action described by behaviour using the provided arguments as the values of the corresponding parameters specified by behaviour. The newfunction objecthas internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use theordinary objectinternal methods except where it is explicitly specified otherwise below:
An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in10.1. These methods are installed inBoundFunctionCreate.
Bound function exotic objects do not have the internal slots of ECMAScript function objects listed inTable 33. Instead they have the internal slots listed inTable 34, in addition to [[Prototype]] and [[Extensible]].
Table 34: Internal Slots of Bound Function Exotic Objects
The [[Construct]] internal method of abound function exotic objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:
The abstract operation BoundFunctionCreate takes arguments targetFunction, boundThis, and boundArgs. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.
10.4.2 Array Exotic Objects
An Array object is anexotic objectthat gives special treatment toarray indexproperty keys (see6.1.7). A property whoseproperty nameis anarray indexis also called an element. Every Array object has a non-configurable"length"property whose value is always a non-negativeintegral Numberwhosemathematical valueis less than 232. The value of the"length"property is numerically greater than the name of every own property whose name is anarray index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is anarray index, the value of the"length"property is changed, if necessary, to be one more than the numeric value of thatarray index; and whenever the value of the"length"property is changed, every own property whose name is anarray indexwhose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array object and is unaffected by"length"orarray indexproperties that may be inherited from its prototypes.
An object is an Array exotic object (or simply, an Array object) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in10.1. These methods are installed inArrayCreate.
10.4.2.1 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of anArray exotic objectA takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:
The abstract operation ArrayCreate takes argument length (a non-negativeinteger) and optional argument proto. It is used to specify the creation of new Array exotic objects. It performs the following steps when called:
The abstract operation ArraySpeciesCreate takes arguments originalArray and length (a non-negativeinteger). It is used to specify the creation of a new Array object using aconstructorfunction that is derived from originalArray. It performs the following steps when called:
If originalArray was created using the standard built-in Arrayconstructorfor arealmthat is not therealmof therunning execution context, then a new Array is created using therealmof therunning execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.
10.4.2.4 ArraySetLength ( A, Desc )
The abstract operation ArraySetLength takes arguments A (an Array object) and Desc (aProperty Descriptor). It performs the following steps when called:
In steps3and4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.
10.4.3 String Exotic Objects
A String object is anexotic objectthat encapsulates a String value and exposes virtualinteger-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have adata propertynamed"length"whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the"length"property are non-writable and non-configurable.
An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in10.1. These methods are installed inStringCreate.
String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.
10.4.3.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of aString exotic objectS takes argument P (a property key). It performs the following steps when called:
The [[DefineOwnProperty]] internal method of aString exotic objectS takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:
For each own property key P of O such that P is anarray indexand ! ToIntegerOrInfinity(P) ≥ len, in ascending numeric index order, do
Add P as the last element of keys.
For each own property key P of O such thatType(P) is String and P is not anarray index, in ascending chronological order of property creation, do
Add P as the last element of keys.
For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
Add P as the last element of keys.
Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and prototype. It is used to specify the creation of new String exotic objects. It performs the following steps when called:
Let S be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at indexℝ(index).
Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]:false, [[Enumerable]]:true, [[Configurable]]:false}.
10.4.4 Arguments Exotic Objects
Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either anordinary objector anarguments exotic object. Anarguments exotic objectis anexotic objectwhosearray indexproperties map to the formal parameters bindings of an invocation of its associated ECMAScript function.
An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in10.1. These methods are installed inCreateMappedArgumentsObject.
Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (20.1.3.6) to identify them as such.
Note 2
Theinteger-indexed data properties of anarguments exotic objectwhose numeric name values are less than the number of formal parameters of the correspondingfunction objectinitially share their values with the corresponding argument bindings in the function'sexecution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into anaccessor property. If the arguments object is anordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
Note 4
Ordinary arguments objects define a non-configurableaccessor propertynamed"callee"which throws aTypeErrorexception on access. The"callee"property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects have historically contained anaccessor propertynamed"caller". Prior to ECMAScript 2017, this specification included the definition of a throwing"caller"property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing"caller"accessor.
10.4.4.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of anarguments exotic objectargs takes argument P (a property key). It performs the following steps when called:
The [[DefineOwnProperty]] internal method of anarguments exotic objectargs takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:
The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (aParse Node), argumentsList (aList), and env (anEnvironment Record). It performs the following steps when called:
Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
Set obj.[[GetOwnProperty]] as specified in10.4.4.1.
Set obj.[[DefineOwnProperty]] as specified in10.4.4.2.
The abstract operation MakeArgGetter takes arguments name (a String) and env (anEnvironment Record). It creates a built-infunction objectthat when executed returns the value bound for name in env. It performs the following steps when called:
Let getterClosure be a newAbstract Closurewith no parameters that captures name and env and performs the following steps when called:
NOTE: getter is never directly accessible to ECMAScript code.
Return getter.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and env (anEnvironment Record). It creates a built-infunction objectthat when executed sets the value bound for name in env. It performs the following steps when called:
Let setterClosure be a newAbstract Closurewith parameters (value) that captures name and env and performs the following steps when called:
Integer-Indexed exotic objectshave the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.
An object is an Integer-Indexed exotic object if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in10.1. These methods are installed byIntegerIndexedObjectCreate.
10.4.5.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of anInteger-Indexed exotic objectO takes argument P (a property key). It performs the following steps when called:
The [[HasProperty]] internal method of anInteger-Indexed exotic objectO takes argument P (a property key). It performs the following steps when called:
For each own property key P of O such thatType(P) is String and P is not aninteger index, in ascending chronological order of property creation, do
Add P as the last element of keys.
For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
Add P as the last element of keys.
Return keys.
10.4.5.8 IntegerIndexedObjectCreate ( prototype )
The abstract operation IntegerIndexedObjectCreate takes argument prototype. It is used to specify the creation of newInteger-Indexed exotic objects. It performs the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.
10.4.6 Module Namespace Exotic Objects
Amodule namespace exotic objectis anexotic objectthat exposes the bindings exported from an ECMAScriptModule(See16.2.3). There is a one-to-one correspondence between the String-keyed own properties of amodule namespace exotic objectand the binding names exported by theModule. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is theStringValueof the corresponding exported binding name. These are the only String-keyed properties of amodule namespace exotic object. Each such property has the attributes { [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:false}. Module namespace exotic objects are not extensible.
An object is a module namespace exotic object if its [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in10.1. These methods are installed byModuleNamespaceCreate.
Module namespace exotic objects have the internal slots defined inTable 35.
Table 35: Internal Slots of Module Namespace Exotic Objects
AListwhose elements are the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using %Array.prototype.sort% usingundefinedas comparefn.
[[Prototype]]
Null
This slot always contains the valuenull(see10.4.6.1).
Module namespace exotic objects provide alternative definitions for all of the internal methods except [[GetPrototypeOf]], which behaves as defined in10.1.1.
10.4.6.1 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of amodule namespace exotic objectO takes argument V (an Object ornull). It performs the following steps when called:
The [[IsExtensible]] internal method of amodule namespace exotic objecttakes no arguments. It performs the following steps when called:
Returnfalse.
10.4.6.3 [[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of amodule namespace exotic objecttakes no arguments. It performs the following steps when called:
Returntrue.
10.4.6.4 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of amodule namespace exotic objectO takes argument P (a property key). It performs the following steps when called:
If Desc.[[Writable]] is present and has valuefalse, returnfalse.
If Desc.[[Value]] is present, returnSameValue(Desc.[[Value]], current.[[Value]]).
Returntrue.
10.4.6.6 [[HasProperty]] ( P )
The [[HasProperty]] internal method of amodule namespace exotic objectO takes argument P (a property key). It performs the following steps when called:
ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of eachmodule namespace exotic object.
The abstract operation ModuleNamespaceCreate takes arguments module and exports. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:
Set M's essential internal methods to the definitions specified in10.4.6.
Set M.[[Prototype]] tonull.
Set M.[[Module]] to module.
Let sortedExports be aListwhose elements are the elements of exports ordered as if an Array of the same values had been sorted using %Array.prototype.sort% usingundefinedas comparefn.
Set M.[[Exports]] to sortedExports.
Create own properties of M corresponding to the definitions in28.3.
An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specificimmutable prototype exotic objectin question.)
Note
Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by%Object.prototype%and byhostenvironments, and inhostenvironments, the relevant objects are potentially exotic in other ways and thus need their own dedicated creation operation.
10.4.7.1 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of animmutable prototype exotic objectO takes argument V (an Object ornull). It performs the following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A proxy object is anexotic objectwhose essential internal methods are partially implemented using ECMAScript code. Every proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, ornull. Methods (seeTable 36) of a handler object may be used to augment the implementation for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or thenullvalue. This object is called the proxy's target object.
An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed inProxyCreate.
Table 36: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set tonullcausing subsequent invocations of internal methods on that proxy object to throw aTypeErrorexception.
Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in6.1.7.3. Some of the internal method invariants defined in6.1.7.3are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.
In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is anyECMAScript language valueand Desc is aProperty Descriptorrecord.
10.5.1 [[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:
[[GetPrototypeOf]] for proxy objects enforces the following invariants:
The result of [[GetPrototypeOf]] must be either an Object ornull.
If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.
10.5.2 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of aProxy exotic objectO takes argument V (an Object ornull). It performs the following steps when called:
[[IsExtensible]] for proxy objects enforces the following invariants:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.
10.5.4 [[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:
If targetDesc isundefinedor targetDesc.[[Configurable]] istrue, then
Throw aTypeErrorexception.
If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] isfalse, then
If targetDesc.[[Writable]] istrue, throw aTypeErrorexception.
Return resultDesc.
Note
[[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object orundefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if the target object is not extensible, unless it does not exist as an own property of the target object.
A property cannot be reported as existent, if the target object is not extensible, unless it exists as an own property of the target object.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.
10.5.6 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of aProxy exotic objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:
If settingConfigFalse istrueand targetDesc.[[Configurable]] istrue, throw aTypeErrorexception.
IfIsDataDescriptor(targetDesc) istrue, targetDesc.[[Configurable]] isfalse, and targetDesc.[[Writable]] istrue, then
If Desc has a [[Writable]] field and Desc.[[Writable]] isfalse, throw aTypeErrorexception.
Returntrue.
Note
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying theProperty Descriptorof the property to the target object using [[DefineOwnProperty]] will not throw an exception.
10.5.7 [[HasProperty]] ( P )
The [[HasProperty]] internal method of aProxy exotic objectO takes argument P (a property key). It performs the following steps when called:
If extensibleTarget isfalse, throw aTypeErrorexception.
Return booleanTrapResult.
Note
[[HasProperty]] for proxy objects enforces the following invariants:
The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
10.5.8 [[Get]] ( P, Receiver )
The [[Get]] internal method of aProxy exotic objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:
IfIsAccessorDescriptor(targetDesc) istrueand targetDesc.[[Get]] isundefined, then
If trapResult is notundefined, throw aTypeErrorexception.
Return trapResult.
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable owndata property.
The value reported for a property must beundefinedif the corresponding target object property is a non-configurable ownaccessor propertythat hasundefinedas its [[Get]] attribute.
If targetDesc.[[Set]] isundefined, throw aTypeErrorexception.
Returntrue.
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable owndata property.
Cannot set the value of a property if the corresponding target object property is a non-configurable ownaccessor propertythat hasundefinedas its [[Set]] attribute.
10.5.10 [[Delete]] ( P )
The [[Delete]] internal method of aProxy exotic objectO takes argument P (a property key). It performs the following steps when called:
The Type of each resultListelement is either String or Symbol.
The resultListmust contain the keys of all non-configurable own properties of the target object.
If the target object is not extensible, then the resultListmust contain all the keys of the own properties of the target object and no other values.
10.5.12 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of aProxy exotic objectO takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:
AProxy exotic objectonly has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
The [[Construct]] internal method of aProxy exotic objectO takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
IfType(newObj) is not Object, throw aTypeErrorexception.
Return newObj.
Note 1
AProxy exotic objectonly has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for proxy objects enforces the following invariants:
The result of [[Construct]] must be an Object.
10.5.14 ProxyCreate ( target, handler )
The abstract operation ProxyCreate takes arguments target and handler. It is used to specify the creation of new Proxy exotic objects. It performs the following steps when called:
IfType(target) is not Object, throw aTypeErrorexception.
IfType(handler) is not Object, throw aTypeErrorexception.
Let P be ! MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in10.5.
ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence ofSourceCharactervalues, eachSourceCharacterbeing a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points). It converts text into a String value, as described in6.1.4. It performs the following steps when called:
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit). Two code units that form a UTF-16surrogate pairare converted to a code point. It performs the following steps when called:
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string, position )
The abstract operation CodePointAt takes arguments string (a String) and position (a non-negativeinteger). It interprets string as a sequence of UTF-16 encoded code points, as described in6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:
The abstract operation StringToCodePoints takes argument string (a String). It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in6.1.4. It performs the following steps when called:
The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars). It performs the following steps when called:
If the parse succeeded and no early errors were found, return theParse Node(an instance of goalSymbol) at the root of the parse tree resulting from the parse.
Otherwise, return aListof one or moreSyntaxErrorobjects representing the parsing errors and/or early errors. If more than one parsing error orearly erroris present, the number and ordering of error objects in the list isimplementation-defined, but at least one must be present.
Note 1
Consider a text that has anearly errorat a particular point, and also a syntax error at a later point. An implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed to the early errors pass. An implementation that interleaves the two activities might report theearly errorand not proceed to find the syntax error. A third implementation might report both errors. All of these behaviours are conformant.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScriptScript. The eval code for a particular invocation of eval is the global code portion of thatScript.
then the source text matching theBindingIdentifier(if any) of that declaration or expression is also included in the function code of the corresponding function.
Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Functionconstructor(20.2.1.1), the GeneratorFunctionconstructor(27.3.1.1), and the AsyncFunctionconstructor(27.7.1.1).
Note 2
The practical effect of including theBindingIdentifierin function code is that the Early Errors forstrict mode codeare applied to aBindingIdentifierthat is the name of a function whose body contains a "use strict" directive, even if the surrounding code is notstrict mode code.
11.2.1 Directive Prologues and the Use Strict Directive
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is aFunctionBodythat begins with aDirective Prologuethat contains aUse Strict Directive.
ECMAScript code that is not strict mode code is called non-strict code.
11.2.3 Non-ECMAScript Functions
An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in somehost-definedform of executable code other than via ECMAScript code. Whether afunction objectis an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScriptScriptorModuleis first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leadingRegularExpressionLiteralare permitted. This is not affected by semicolon insertion (see12.9); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after aLineTerminatoris U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at theLineTerminator. That is, the above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in anIdentifierNameafter the first character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see12.2).
The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized inTable 37.
White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within aStringLiteral, aRegularExpressionLiteral, aTemplate, or aTemplateSubstitutionTailwhere they are considered significant code points forming part of a literal value. They may also occur within aComment, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed inTable 38.
Table 38: White Space Code Points
Code Point
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+0020
SPACE
<SP>
U+00A0
NO-BREAK SPACE
<NBSP>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
Other category “Zs”
Any other Unicode “Space_Separator” code point
<USP>
ECMAScript implementations must recognize asWhiteSpacecode points listed in the “Space_Separator” (“Zs”) category.
Note
Other than for the code points listed inTable 38, ECMAScriptWhiteSpaceintentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (12.9). A line terminator cannot occur within any token except aStringLiteral,Template, orTemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within aStringLiteraltoken except as part of aLineContinuation.
Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed inTable 39.
Table 39: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points inTable 39are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed inTable 38. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a singleSourceCharacterfor the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except aLineTerminatorcode point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, theLineTerminatorat the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see12.9).
Comments behave like white space and are discarded except that, if aMultiLineCommentcontains a line terminator code point, then the entire comment is considered to be aLineTerminatorfor purposes of parsing by the syntactic grammar.
IdentifierNameandReservedWordare tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications.ReservedWordis an enumerated subset ofIdentifierName. The syntactic grammar definesIdentifieras anIdentifierNamethat is not aReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in anIdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of anIdentifierName.
TwoIdentifierNames that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of eachUnicodeEscapeSequence, they are represented by the exact same sequence of code points.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
A keyword is a token that matchesIdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.
A reserved word is anIdentifierNamethat cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.
This specification uses a combination of grammatical productions andearly errorrules to specify which names are valid identifiers and which are reserved words. All tokens in theReservedWordlist below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in13.1, using parameterized syntactic productions. Lastly, severalearly errorrules restrict the set of valid identifiers. See13.1.1,14.3.1.1,14.7.5.1, and15.7.1. In summary, there are five categories of identifier names:
Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;
Those that are never allowed as identifiers, namely theReservedWords listed below except await and yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, instrict mode code: let, static, implements, interface, package, private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places whereIdentifieris not allowed: as, async, from, get, meta, of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.
Per5.1.5, keywords in the grammar match literal sequences of specificSourceCharacterelements. A code point in a keyword cannot be expressed by a \UnicodeEscapeSequence.
enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private, protected, and public are future reserved words instrict mode code.
A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in11.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal, except as part of aLineContinuationto produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.
12.8.4.1 Static Semantics: SV
A string literal stands for a value of the String type. SV produces String values for string literals through recursive application on the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having amathematical value, as described below or in12.8.3.
A regular expression literal is an input element that is converted to a RegExp object (see22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExpconstructoras a function (see22.2.3).
The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising theRegularExpressionBodyand theRegularExpressionFlagsare subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in22.2.1, but it must not extend theRegularExpressionBodyandRegularExpressionFlagsproductions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.
A template literal component is interpreted by TV as a value of the String type. TV is used to construct the indexed components of a template object (colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence.
A template literal component is interpreted by TRV as a value of the String type. TRV is used to construct the raw components of a template object (colloquially, the template raw values). TRV is similar toTVwith the difference being that in TRV, escape sequences are interpreted as they appear in the literal.
The TRV ofLineTerminatorSequence::<LF>is the String value consisting of the code unit 0x000A (LINE FEED).
The TRV ofLineTerminatorSequence::<CR>is the String value consisting of the code unit 0x000A (LINE FEED).
The TRV ofLineTerminatorSequence::<LS>is the String value consisting of the code unit 0x2028 (LINE SEPARATOR).
The TRV ofLineTerminatorSequence::<PS>is the String value consisting of the code unit 0x2029 (PARAGRAPH SEPARATOR).
The TRV ofLineTerminatorSequence::<CR><LF>is the String value consisting of the code unit 0x000A (LINE FEED).
Note
TVexcludes the code units ofLineContinuationwhile TRV includes them. <CR><LF> and <CR>LineTerminatorSequences are normalized to <LF> for bothTVand TRV. An explicitEscapeSequenceis needed to include a <CR> or <CR><LF> sequence.
12.9 Automatic Semicolon Insertion
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.
12.9.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current lexicalgoal symbolas described in clause12.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
The offending token is separated from the previous token by at least oneLineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[noLineTerminatorhere]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least oneLineTerminator, then a semicolon is automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see14.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least oneLineTerminatoroccurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and aLineTerminatoris encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should appear on the same line as its operand.
AnExpressionin a return or throw statement or anAssignmentExpressionin a yield expression should start on the same line as the return, throw, or yield token.
ALabelIdentifierin a break or continue statement should be on the same line as the break or continue token.
12.9.2 Examples of Automatic Semicolon Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return statement, because aLineTerminatorseparates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b, because aLineTerminatoroccurs between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.
12.9.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.
12.9.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists
In aStatementList, manyStatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:
An opening parenthesis ((). Without a semicolon, the two lines together are treated as aCallExpression.
An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than anArrayLiteralorArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11), with the previous expression as theMemberExpression.
Unary + or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the /MultiplicativeOperator, for example if the RegExp has flags.
12.9.3.2 Cases of Automatic Semicolon Insertion and “[noLineTerminatorhere]”
This section is non-normative.
ECMAScript contains grammar productions which include “[noLineTerminatorhere]”. These productions are sometimes a means to have optional operands in the grammar. Introducing aLineTerminatorin these locations would change the grammar production of a source text by using the grammar production without the optional operand.
The rest of this section describes a number of productions using “[noLineTerminatorhere]” in this version of ECMAScript.
12.9.3.2.1 List of Grammar Productions with Optional Operands and “[noLineTerminatorhere]”
yield and await are permitted asBindingIdentifierin the grammar, and prohibited withstatic semanticsbelow, to prohibit automatic semicolon insertion in cases such as
It is a Syntax Error if this phrase is contained instrict mode codeand theStringValueofIdentifierNameis:"implements","interface","let","package","private","protected","public","static", or"yield".
AnArrayLiteralis an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by anAssignmentExpression(i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.
CreateDataPropertyOrThrowis used to ensure that own properties are defined for the array even if the standard built-inArray prototype objecthas been modified in a manner that would preclude the creation of new own properties using [[Set]].
An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.
In certain contexts,ObjectLiteralis used as a cover grammar for a more restricted secondary grammar. TheCoverInitializedNameproduction is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actualObjectLiteralis expected.
The abstract operation IsValidRegularExpressionLiteral takes argument literal. It determines if its argument is a valid regular expression literal. It performs the following steps when called:
Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
Append theRecord{ [[Site]]: templateLiteral, [[Array]]: template } to templateRegistry.
Return template.
Note 1
The creation of a template object cannot result in anabrupt completion.
Note 2
EachTemplateLiteralin the program code of arealmis associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of theTemplateLiteralor eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
Return the result of evaluatingExpression. This may be of type Reference.
Note
This algorithm does not applyGetValueto the result of evaluatingExpression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.
The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (anECMAScript language value), expression (aParse Node), and strict (a Boolean). It performs the following steps when called:
Let propertyNameReference be the result of evaluating expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (anECMAScript language value), identifierName (aParse Node), and strict (a Boolean). It performs the following steps when called:
Assert: If tailPosition istrue, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
A tagged template is a function call where the arguments of the call are derived from aTemplateLiteral(13.2.8). The actual arguments include a template object (13.2.8.3) and the values produced by evaluating the expressions embedded within theTemplateLiteral.
Thehost-definedabstract operation HostGetImportMetaProperties takes argument moduleRecord (aModule Record). It allows hosts to provide property keys and values for the object returned from import.meta.
An implementation of HostGetImportMetaProperties must conform to the following requirements:
It must return anormal completionwith a value of aListwhose values are all Records with two fields, [[Key]] and [[Value]].
Each suchRecord's [[Key]] field must be a property key, i.e.,IsPropertyKeymust returntruewhen applied to it.
Each suchRecord's [[Value]] field must be an ECMAScript value.
The default implementation of HostGetImportMetaProperties is to returnNormalCompletion(« »).
Thehost-definedabstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (aModule Record). It allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.
Most hosts will be able to simply defineHostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.
An implementation of HostFinalizeImportMeta must conform to the following requirements:
When a delete operator occurs withinstrict mode code, aSyntaxErrorexception is thrown if itsUnaryExpressionis a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs withinstrict mode codeand the property to be deleted has the attribute { [[Configurable]]:false} (or otherwise cannot be deleted), aTypeErrorexception is thrown.
Note 2
The object that may be created in step5.cis not accessible outside of the above abstract operation and theordinary object[[Delete]] internal method. An implementation might choose to avoid the actual creation of that object.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator takes arguments V (anECMAScript language value) and target (anECMAScript language value). It implements the generic algorithm for determining if V is an instance of target either by consulting target's@@hasInstancemethod or, if absent, determining whether the value of target's"prototype"property is present in V's prototype chain. It performs the following steps when called:
IfType(target) is not Object, throw aTypeErrorexception.
Steps4and5provide compatibility with previous editions of ECMAScript that did not use a@@hasInstancemethod to define the instanceof operator semantics. If an object does not define or inherit@@hasInstanceit uses the default instanceof semantics.
13.11 Equality Operators
Note
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a" and "a" == new String("a") are bothtrue.
new String("a") == new String("a") isfalse.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for aConditionalExpressionin ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be anExpressionbut restrict the third expression to be aConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
When this expression occurs withinstrict mode code, it is a runtime error if lref in step1.e,2,2,2,2is an unresolvable reference. If it is, aReferenceErrorexception is thrown. Additionally, it is a runtime error if the lref in step8,7,7,6is a reference to adata propertywith the attribute value { [[Writable]]:false}, to anaccessor propertywith the attribute value { [[Set]]:undefined}, or to a non-existent property of an object for which theIsExtensiblepredicate returns the valuefalse. In these cases aTypeErrorexception is thrown.
The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (anECMAScript language value), opText (a sequence of Unicode code points), and rval (anECMAScript language value). It performs the following steps when called:
Let operation be the abstract operation associated with opText in the following table:
opText
operation
**
T::exponentiate
*
T::multiply
/
T::divide
%
T::remainder
+
T::add
-
T::subtract
<<
T::leftShift
>>
T::signedRightShift
>>>
T::unsignedRightShift
&
T::bitwiseAND
^
T::bitwiseXOR
|
T::bitwiseOR
Return ? operation(lnum, rnum).
Note 1
No hint is provided in the calls toToPrimitivein steps2.aand2.b. All standard objects except Date objects handle the absence of a hint as ifnumberwere given; Date objects handle the absence of a hint as ifstringwere given. Exotic objects may handle the absence of a hint in some other manner.
Note 2
Step2.cdiffers from step3of theIsLessThanalgorithm, by using the logical-or operation instead of the logical-and operation.
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (aParse Node), opText (a sequence of Unicode code points), and rightOperand (aParse Node). It performs the following steps when called:
Left to right evaluation order is maintained by evaluating aDestructuringAssignmentTargetthat is not a destructuring pattern prior to accessing the iterator or evaluating theInitializer.
The value of aStatementListis the value of the last value-producing item in theStatementList. For example, the following calls to the eval function all return the value 1:
The abstract operation BlockDeclarationInstantiation takes arguments code (aParse Node) and env (anEnvironment Record). code is theParse Nodecorresponding to the body of the block. env is theEnvironment Recordin which bindings are to be created.
With parameters value, environment, and propertyName.
Note
Whenundefinedis passed for environment it indicates that aPutValueoperation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs
The abstract operation ForBodyEvaluation takes arguments test, increment, stmt, perIterationBindings, and labelSet. It performs the following steps when called:
undefinedis passed for environment to indicate that aPutValueoperation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames, expr, and iterationKind (eitherenumerate,iterate, orasync-iterate). It performs the following steps when called:
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind (eitherassignment,varBindingorlexicalBinding), and labelSet and optional argument iteratorKind (eithersyncorasync). It performs the following steps when called:
If iteratorKind is not present, set iteratorKind tosync.
Return an Iterator object (27.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods arenulland are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. Aproperty namewill be returned by the iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in14.7.5.10.2.1directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey ofEnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which implementations are not required to matchCreateForInIteratorwas chosen because implementations historically differed in behaviour for those cases, and agreed in all others.
14.7.5.10 For-In Iterator Objects
A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour ofEnumerateObjectProperties.
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:
14.7.5.10.3 Properties of For-In Iterator Instances
For-In Iterator instances are ordinary objects that inherit properties from the%ForInIteratorPrototype%intrinsic object. For-In Iterator instances are initially created with the internal slots listed inTable 42.
Table 42: Internal Slots of For-In Iterator Instances
Internal Slot
Description
[[Object]]
The Object value whose properties are being iterated.
[[ObjectWasVisited]]
trueif the iterator has invoked [[OwnPropertyKeys]] on [[Object]],falseotherwise.
[[VisitedKeys]]
A list of String values which have been emitted by this iterator thus far.
[[RemainingKeys]]
A list of String values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is notnull).
It is a Syntax Error if thisContinueStatementis not nested, directly or indirectly (but not crossing function boundaries), within anIterationStatement.
A return statement causes a function to cease execution and, in most cases, returns a value to the caller. IfExpressionis omitted, the return value isundefined. Otherwise, the return value is the value ofExpression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.
The with statement adds anobject Environment Recordfor a computed object to the lexical environment of therunning execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
No matter how control leaves the embeddedStatement, whether normally or by some form ofabrupt completionor exception, the LexicalEnvironment is always restored to its former state.
The abstract operation CaseClauseIsSelected takes arguments C (aParse NodeforCaseClause) and input (anECMAScript language value). It determines whether C matches input. It performs the following steps when called:
This operation does not execute C'sStatementList(if any). TheCaseBlockalgorithm uses its return value to determine whichStatementListto start executing.
AStatementmay be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. AStatementcan be part of aLabelledStatement, which itself can be part of aLabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, itsCatchParameteris bound to that exception.
Evaluating aDebuggerStatementmay allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function objects (10.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (10.2.1).
The ExpectedArgumentCount of aFormalParameterListis the number ofFormalParametersto the left of either the rest parameter or the firstFormalParameterwith an Initializer. AFormalParameterwithout an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional withundefinedas their default value.
A"prototype"property is automatically created for every function defined using aFunctionDeclarationorFunctionExpression, to allow for the possibility that the function will be used as aconstructor.
AnArrowFunctiondoes not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within anArrowFunctionmust resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though anArrowFunctionmay contain references to super, thefunction objectcreated in step5is not made into a method by performingMakeMethod. AnArrowFunctionthat references super is always contained within a non-ArrowFunctionand the necessary state to implement super is accessible via the scope that is captured by thefunction objectof theArrowFunction.
YieldExpressioncannot be used within theFormalParametersof a generator function because any expressions that are part ofFormalParametersare evaluated before the resulting generator object is in a resumable state.
Let G be ? OrdinaryCreateFromConstructor(functionObject,"%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
If generatorKind isasync, set innerResult to ? Await(innerResult).
NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
IfType(innerResult) is not Object, throw aTypeErrorexception.
NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
Let closeCompletion beCompletion{ [[Type]]:normal, [[Value]]:empty, [[Target]]:empty}.
If generatorKind isasync, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
It is a Syntax Error ifPrivateBoundIdentifiersofClassElementListcontains any duplicate entries, unless the name is used once for a getter and once for a setter and in no other entries, and the getter and setter are either both static or both non-static.
Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of AllPrivateIdentifiersValid:
Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsArguments:
For ease of specification, private methods and accessors are included alongside private fields in the [[PrivateElements]] slot of class instances. However, any given object has either all or none of the private methods and accessors defined by a given class. This feature has been designed so that implementations may choose to implement private methods and accessors using a strategy which does not require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods with their correspondingPrivate Nameand track, for each object, which class constructors have run with that object as their this value. Looking up an instance private method on an object then consists of checking that the classconstructorwhich defines the method has been used to initialize the object, then returning the method associated with thePrivate Name.
This differs from private fields: because field initializers can throw during class instantiation, an individual object may have some proper subset of the private fields of a given class, and so private fields must in general be tracked individually.
NOTE: This branch behaves similarly to constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the@@iteratormethod on %Array.prototype%, this function does not.
WhenModuleis the syntacticgoal symboland the [Await] parameter is absent, await is parsed as akeywordand will be a Syntax error. WhenScriptis the syntacticgoal symbol, await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:
Tail Position calls are only defined instrict mode codebecause of a common non-standard language extension (see10.2.4) that enables observation of the chain of caller contexts.
15.10.2 Static Semantics: HasCallInTailPosition
With parameter call.
Note
call is aParse Nodethat represents a specific range of source text. When the following algorithms compare call to anotherParse Node, it is a test of whether they represent the same source text.
A potential tail position call that is immediately followed by returnGetValueof the call result is also a possible tail position call. A function call cannot return aReference Record, so such aGetValueoperation will always return the same value as the actual function call result.
A tail position call must either release any transient internal resources associated with the currently executing functionexecution contextbefore invoking the target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.
The abstract operation ParseScript takes arguments sourceText, realm, and hostDefined. It creates aScript Recordbased upon the result of parsing sourceText as aScript. It performs the following steps when called:
Assert: sourceText is an ECMAScript source text (see clause11).
An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.
16.1.6 ScriptEvaluation ( scriptRecord )
The abstract operation ScriptEvaluation takes argument scriptRecord. It performs the following steps when called:
Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
The abstract operation GlobalDeclarationInstantiation takes arguments script (aParse NodeforScriptBody) and env (anEnvironment Record). script is theScriptBodyfor which theexecution contextis being established. env is the global environment in which bindings are to be created.
Note 1
When anexecution contextis established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.
If vn is not an element of declaredFunctionNames, then
Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
If vnDefinable isfalse, throw aTypeErrorexception.
If vn is not an element of declaredVarNames, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if theglobal objectis anordinary object. However, if theglobal objectis aProxy exotic objectit may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: AnnexB.3.2.2adds additional steps at this point.
Early errors specified in16.1.1prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a singleScript. However, such conflicts and redeclarations that span more than oneScriptare detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if theglobal objectis defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in anabrupt completionand some global declarations not being instantiated. If this occurs, the code for theScriptis not evaluated.
Unlike explicit var or function declarations, properties that are directly created on theglobal objectresult in global bindings that may be shadowed by let/const/class declarations.
The duplicateExportedNamesrule implies that multiple export defaultExportDeclarationitems within aModuleBodyis a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of aModule. If any such errors are detected theModuleis not evaluated.
The abstract operation ImportedLocalNames takes argument importEntries (aListof ImportEntry Records (seeTable 49)). It creates aListof all of the local name bindings defined by importEntries. It performs the following steps when called:
A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.
For specification purposes Module Record values are values of theRecordspecification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with both abstract and concrete subclasses. This specification defines the abstract subclass namedCyclic Module Recordand its concrete subclass namedSource Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.
Module Record defines the fields listed inTable 44. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list inTable 45. All Module definition subclasses must provide concrete implementations of these abstract methods.
TheEnvironment Recordcontaining the top level bindings for this module. This field is set when the module is linked.
[[Namespace]]
Object |undefined
The Module Namespace Object (28.3) if one has been created for this module. Otherwiseundefined.
[[HostDefined]]
Any, default value isundefined.
Field reserved for use byhostenvironments that need to associate additional information with a module.
Table 45: Abstract Methods of Module Records
Method
Purpose
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly exported from this module.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]:Module Record, [[BindingName]]: String }. If the export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to"*namespace*". Returnnullif the name cannot be resolved, or"ambiguous"if multiple bindings were found.
Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result if it completes normally.
Link()
Prepare the module for evaluation by transitively resolving all module dependencies and creating amodule Environment Record.
Evaluate()
If this module has already been evaluated successfully, returnundefined; if it has already been evaluated unsuccessfully, throw the exception that was produced. Otherwise, transitively evaluate all module dependencies of this module and then evaluate this module.
Link must have completed successfully prior to invoking this method.
16.2.1.5 Cyclic Module Records
A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of theCyclic Module Recordtype. Module Records that are not subclasses of theCyclic Module Recordtype must not participate in dependency cycles with Source Text Module Records.
In addition to the fields defined inTable 44Cyclic Module Records have the additional fields listed inTable 46
Table 46: Additional Fields of Cyclic Module Records
Field Name
Value Type
Meaning
[[Status]]
unlinked|linking|linked|evaluating|evaluated
Initiallyunlinked. Transitions tolinking,linked,evaluating,evaluated(in that order) as the module progresses throughout its lifecycle.
A completion of typethrowrepresenting the exception that occurred during evaluation.undefinedif no exception occurred or if [[Status]] is notevaluated.
Auxiliary field used during Link and Evaluate only. If [[Status]] islinkingorevaluating, this non-negative number records the point at which the module was first visited during the ongoing depth-first traversal of the dependency graph.
Auxiliary field used during Link and Evaluate only. If [[Status]] islinkingorevaluating, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
AListof all theModuleSpecifierstrings used by the module represented by this record to request the importation of a module. TheListis source code occurrence ordered.
In addition to the methods defined inTable 45Cyclic Module Records have the additional methods listed inTable 47
Table 47: Additional Abstract Methods of Cyclic Module Records
The Link concrete method of aCyclic Module Recordmodule takes no arguments. On success, Link transitions this module's [[Status]] fromunlinkedtolinked. On failure, an exception is thrown and this module's [[Status]] remainsunlinked. (Most of the work is done by the auxiliary functionInnerModuleLinking.) It performs the following steps when called:
Assert: module.[[Status]] is notlinkingorevaluating.
16.2.1.5.1.1 InnerModuleLinking ( module, stack, index )
The abstract operation InnerModuleLinking takes arguments module (aCyclic Module Record), stack, and index (a non-negativeinteger). It is used by Link to perform the actual linking process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition tolinkedtogether. It performs the following steps when called:
If requiredModule and module are the sameModule Record, set done totrue.
Return index.
16.2.1.5.2 Evaluate ( )
The Evaluate concrete method of aCyclic Module Recordmodule takes no arguments. Evaluate transitions this module's [[Status]] fromlinkedtoevaluated. If execution results in an exception, that exception is recorded in the [[EvaluationError]] field and rethrown by future invocations of Evaluate. (Most of the work is done by the auxiliary functionInnerModuleEvaluation.) It performs the following steps when called:
Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within thesurrounding agent.
16.2.1.5.2.1 InnerModuleEvaluation ( module, stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (aModule Record), stack, and index (a non-negativeinteger). It is used by Evaluate to perform the actual evaluation process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as inInnerModuleLinking. It performs the following steps when called:
If requiredModule and module are the sameModule Record, set done totrue.
Return index.
16.2.1.5.3 Example Cyclic Module Record Graphs
This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When ahostfirst calls A.Link(), this will complete successfully by assumption, and recursively link modules B and C as well, such that A.[[Status]] = B.[[Status]] = C.[[Status]] =linked. This preparatory step can be performed at any time. Later, when thehostis ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will beevaluated.
Consider then cases involving linking errors. IfInnerModuleLinkingof C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Link() will fail, and both A and B's [[Status]] remainunlinked. C's [[Status]] has becomelinked, though.
Finally, consider a case involving evaluation errors. IfInnerModuleEvaluationof C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will becomeevaluated. C will also becomeevaluatedbut, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time ahosttries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)
The difference here between linking and evaluation errors is due to how evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.) Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but noModule Recordexists for that module, i.e.HostResolveImportedModulethrows an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing butParseModulethrowing an exception when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the exception they throw fromHostResolveImportedModule. In any case, this exception causes a linking failure, which as before results in A's [[Status]] remainingunlinked.
Lastly, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that thehostproceeds by calling A.Link(), which performsInnerModuleLinkingon A. This in turn callsInnerModuleLinkingon B. Because of the cycle, this again triggersInnerModuleLinkingon A, but at this point it is a no-op since A.[[Status]] is alreadylinking. B.[[Status]] itself remainslinkingwhen control gets back to A andInnerModuleLinkingis triggered on C. After this returns with C.[[Status]] beinglinked, both A and B transition fromlinkingtolinkedtogether; this is by design, since they form a strongly connected component.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where A has an linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call toInnerModuleLinkingon A. However, once we unwind back to the originalInnerModuleLinkingon A, it fails during InitializeEnvironment, namely right after C.ResolveExport(). The thrownSyntaxErrorexception propagates up to A.Link, which resets all modules that are currently on its stack (these are always exactly the modules that are stilllinking). Hence both A and B becomeunlinked. Note that C is left aslinked.
Finally, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call toInnerModuleEvaluationon A. However, once we unwind back to the originalInnerModuleEvaluationon A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are stillevaluating). Hence both A and B becomeevaluatedand the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left asevaluatedwith no [[EvaluationError]].
16.2.1.6 Source Text Module Records
A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (11) that was parsed using thegoal symbolModule. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, link, and evaluate the module.
In addition to the fields defined inTable 46, Source Text Module Records have the additional fields listed inTable 48. Each of these fields is initially set inParseModule.
Table 48: Additional Fields of Source Text Module Records
AListof ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module or exports from export * as namespace declarations.
AListof ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module, not including export * as namespace declarations.
An ImportEntry Record is aRecordthat digests information about a single declarative import. EachImportEntry Recordhas the fields defined inTable 49:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value"*"indicates that the import request is for the target module's namespace object.
[[LocalName]]
String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 50gives examples of ImportEntry records fields used to represent the syntactic import forms:
Table 50 (Informative): Import Forms Mappings to ImportEntry Records
The name under which the desired binding is exported by the module identified by [[ModuleRequest]].nullif theExportDeclarationdoes not have aModuleSpecifier."*"indicates that the export request is for all exported bindings.
[[LocalName]]
String | null
The name that is used to locally access the exported value from within the importing module.nullif the exported value is not locally accessible from within the module.
Note 2
Table 52gives examples of the ExportEntry record fields used to represent the syntactic export forms:
Table 52 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f() {}
"default"
null
null
"f"
export default function () {}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
export * as ns from "mod";
"ns"
"mod"
"*"
null
The following definitions specify the required concrete methods and otherabstract operationsfor Source Text Module Records
The abstract operation ParseModule takes arguments sourceText (ECMAScript source text), realm, and hostDefined. It creates aSource Text Module Recordbased upon the result of parsing sourceText as aModule. It performs the following steps when called:
Assert: sourceText is an ECMAScript source text (see clause11).
An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.
16.2.1.6.2 GetExportedNames ( [ exportStarSet ] )
The GetExportedNames concrete method of aSource Text Module Recordmodule takes optional argument exportStarSet. It performs the following steps when called:
If exportStarSet is not present, set exportStarSet to a new emptyList.
Assert: exportStarSet is aListof Source Text Module Records.
If exportStarSet contains module, then
Assert: We've reached the starting point of an export * circularity.
The ResolveExport concrete method of aSource Text Module Recordmodule takes argument exportName (a String) and optional argument resolveSet.
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by theModule Recordthis method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specificModule Recordand exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.
If a defining module is found, aResolvedBinding Record{ [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export, unless this is the export of a namespace with no local binding. In this case, [[BindingName]] will be set to"*namespace*". If no definition was found or the request is found to be circular,nullis returned. If the request is found to be ambiguous, the string"ambiguous"is returned.
It performs the following steps when called:
If resolveSet is not present, set resolveSet to a new emptyList.
Assert: resolveSet is aListofRecord{ [[Module]], [[ExportName]] }.
For eachRecord{ [[Module]], [[ExportName]] } r of resolveSet, do
If module and r.[[Module]] are the sameModule RecordandSameValue(exportName, r.[[ExportName]]) istrue, then
If starResolution isnull, set starResolution to resolution.
Else,
Assert: There is more than one * import that includes the requested name.
If resolution.[[Module]] and starResolution.[[Module]] are not the sameModule RecordorSameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) isfalse, return"ambiguous".
Return starResolution.
16.2.1.6.4 InitializeEnvironment ( )
The InitializeEnvironment concrete method of aSource Text Module Recordmodule takes no arguments. It performs the following steps when called:
NOTE: The above call cannot fail because imported module requests are a subset of module.[[RequestedModules]], and these have been resolved earlier in this algorithm.
Thehost-definedabstract operation HostResolveImportedModule takes arguments referencingScriptOrModule (aScript RecordorModule Recordornull) and specifier (aModuleSpecifierString). It provides the concreteModule Recordsubclass instance that corresponds to specifier occurring within the context of the script or module represented by referencingScriptOrModule. referencingScriptOrModule may benullif the resolution is being performed in the context of animport()expression and there is noactive script or moduleat that time.
Note
An example of when referencingScriptOrModule can benullis in a web browserhost. There, if a user clicks on a control given by
there will be noactive script or moduleat the time theimport()expression runs. More generally, this can happen in any situation where thehostpushes execution contexts withnullScriptOrModule components onto theexecution context stack.
An implementation of HostResolveImportedModule must conform to the following requirements:
If it completes normally, the [[Value]] slot of the completion must contain an instance of a concrete subclass ofModule Record.
If aModule Recordcorresponding to the pair referencingScriptOrModule, specifier does not exist or cannot be created, an exception must be thrown.
Each time this operation is called with a specific referencingScriptOrModule, specifier pair as arguments it must return the sameModule Recordinstance if it completes normally.
Multiple different referencingScriptOrModule, specifier pairs may map to the sameModule Recordinstance. The actual mapping semantic ishost-definedbut typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.
Thehost-definedabstract operation HostImportModuleDynamically takes arguments referencingScriptOrModule (aScript RecordorModule Recordornull), specifier (aModuleSpecifierString), and promiseCapability (aPromiseCapability Record). It performs any necessary setup work in order to make available the module corresponding to specifier occurring within the context of the script or module represented by referencingScriptOrModule. referencingScriptOrModule may benullif there is noactive script or modulewhen theimport()expression occurs. It then performsFinishDynamicImportto finish the dynamic import process.
An implementation of HostImportModuleDynamically must conform to the following requirements:
It must returnNormalCompletion(undefined). Success or failure must instead be signaled as discussed below.
Thehost environmentmust conform to one of the two following sets of requirements:
The completion value of any subsequent call toHostResolveImportedModuleafterFinishDynamicImporthas completed, given the arguments referencingScriptOrModule and specifier, must be a module which has already been evaluated, i.e. whose Evaluate concrete method has already been called and returned anormal completion.
If thehost environmenttakes the success path once for a given referencingScriptOrModule, specifier pair, it must always do so for subsequent calls.
The operation must not call promiseCapability.[[Resolve]] or promiseCapability.[[Reject]], but instead must treat promiseCapability as an opaque identifying value to be passed through toFinishDynamicImport.
The actual process performed ishost-defined, but typically consists of performing whatever I/O operations are necessary to allowHostResolveImportedModuleto synchronously retrieve the appropriateModule Record, and then calling its Evaluate concrete method. This might require performing similar normalization asHostResolveImportedModuledoes.
The abstract operation FinishDynamicImport takes arguments referencingScriptOrModule, specifier, promiseCapability (aPromiseCapability Record), and completion. FinishDynamicImport completes the process of a dynamic import originally started by animport()call, resolving or rejecting the promise returned by that call as appropriate according to completion. It is performed byhostenvironments as part ofHostImportModuleDynamically. It performs the following steps when called:
If completion is anabrupt completion, perform ! Call(promiseCapability.[[Reject]],undefined, « completion.[[Value]] »).
The abstract operation GetModuleNamespace takes argument module. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:
The only way GetModuleNamespace can throw is via one of the triggeredHostResolveImportedModulecalls. Unresolvable names are simply excluded from the namespace at this point. They will lead to a real linking error later unless they are all ambiguous star exports that are not explicitly requested anywhere.
Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: localName, [[ExportName]]:"default"}.
Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: localName, [[ExportName]]:"default"}.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in theScriptcontaining the error. The presence of anearly errorprevents the evaluation of the construct. An implementation must report early errors in aScriptas part of parsing thatScriptinParseScript. Early errors in aModuleare reported at the point when theModulewould be evaluated and theModuleis never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as anearly errorany occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in17.1, ahostor implementation may extendScriptsyntax,Modulesyntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExpconstructor) that are allowed to throwSyntaxErrorare permitted to exhibithost-definedbehaviour instead of throwingSyntaxErrorwhen they encounter ahost-definedextension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in17.1, ahostor implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to havehost-definedbehaviour instead of throwing an error (such asReferenceError).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
If an implementation extends anyfunction objectwith an own property named"caller"the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be astrict functionobject. If it is anaccessor property, the function that is the value of the property's [[Get]] attribute must never return astrict functionwhen called.
Neither mapped nor unmapped arguments objects may be created with an own property named"caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in22.2.1andB.1.4must not be extended to recognize any of the source characters A-Z or a-z asIdentityEscape[+U]when the [U] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately follow source text that matches theBindingIdentifiernonterminal symbol.
There are certain built-in objects available whenever an ECMAScriptScriptorModulebegins execution. One, theglobal object, is part of the global environment of the executing program. Others are accessible as initial properties of theglobal objector indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-infunction objectwith the characteristics described in10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the valuetrue. Every built-infunction objecthas a [[Realm]] internal slot whose value is theRealm Recordof therealmfor which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of thatfunction object. For each built-inconstructor, this specification furthermore describes properties of the prototype object of thatconstructorand properties of specific object instances returned by a new expression that invokes thatconstructor.
Unless otherwise specified in the description of a particular function, if a built-in function orconstructoris given fewer arguments than the function is specified to require, the function orconstructorshall behave exactly as if it had been given sufficient additional arguments, each such argument being theundefinedvalue. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “thisvalue” and “NewTarget” have the meanings given in10.3.
Unless otherwise specified in the description of a particular function, if a built-in function orconstructordescribed is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of aTypeErrorexception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-inconstructorhas theFunction prototype object, which is the initial value of the expression Function.prototype (20.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has theObject prototype object, which is the initial value of the expression Object.prototype (20.1.3), as the value of its [[Prototype]] internal slot, except theObject prototype objectitself.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling theCreateBuiltinFunctionabstract operation (10.3.3). The values of the length and name parameters are the initial values of the"length"and"name"properties as discussed below. The values of the prefix parameter are similarly discussed below.
Every built-infunction object, including constructors, has a"length"property whose value is a non-negativeintegral Number. Unless otherwise specified, this value is equal to the number of required parameters shown in the subclause heading for the function description. Optional parameters and rest parameters are not included in the parameter count.
Note 2
For example, thefunction objectthat is the initial value of the"map"property of theArray prototype objectis described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the"length"property of thatfunction objectis1𝔽.
Unless otherwise specified, the"length"property of a built-infunction objecthas the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
Every built-infunction object, including constructors, has a"name"property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. Functions that are identified as anonymous functions use the empty String as the value of the"name"property. For functions that are specified as properties of objects, the name value is theproperty namestring used to access the function. Functions that are specified as get or set accessor functions of built-in properties have"get"or"set"(respectively) passed to the prefix parameter when callingCreateBuiltinFunction.
The value of the"name"property is explicitly specified for each built-in functions whose property key is a Symbol value. If such an explicitly specified value starts with the prefix"get "or"set "and the function for which it is specified is a get or set accessor function of a built-in property, the value without the prefix is passed to the name parameter, and the value"get"or"set"(respectively) is passed to the prefix parameter when callingCreateBuiltinFunction.
Unless otherwise specified, the"name"property of a built-infunction objecthas the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
Every otherdata propertydescribed in clauses19through28and in AnnexB.2has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true} unless otherwise specified.
Everyaccessor propertydescribed in clauses19through28and in AnnexB.2has the attributes { [[Enumerable]]:false, [[Configurable]]:true} unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value,undefined. If only a set accessor is described the get accessor is the default value,undefined.
does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value ishost-defined.
may havehostdefined properties in addition to the properties defined in this specification. This may include a property whose value is the global object itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the"globalThis"property of theglobal objectin aRealm Recordrealm is realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
19.1.2 Infinity
The value of Infinity is+∞𝔽 (see6.1.6.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
19.1.3 NaN
The value of NaN isNaN(see6.1.6.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
19.1.4 undefined
The value of undefined isundefined(see6.1.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
19.2 Function Properties of the Global Object
19.2.1 eval ( x )
The eval function is the %eval% intrinsic object. When the eval function is called with one argument x, the following steps are taken:
NOTE: If direct istrue, runningContext will be theexecution contextthat performed thedirect eval. If direct isfalse, runningContext will be theexecution contextfor the invocation of the eval function.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code isstrict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.
Thehost-definedabstract operation HostEnsureCanCompileStrings takes arguments callerRealm (aRealm Record) and calleeRealm (aRealm Record). It allowshostenvironments to block certain ECMAScript functions which allow developers to compile strings into ECMAScript code.
An implementation of HostEnsureCanCompileStrings must conform to the following requirements:
The abstract operation EvalDeclarationInstantiation takes arguments body, varEnv, lexEnv, privateEnv, and strict. It performs the following steps when called:
An alternative version of this algorithm is described inB.3.4.
19.2.2 isFinite ( number )
The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:
A reliable way for ECMAScript code to test if a value X is aNaNis an expression of the form X !== X. The result will betrueif and only if X is aNaN.
19.2.4 parseFloat ( string )
The parseFloat function produces aNumber valuedictated by interpretation of the contents of the string argument as a decimal literal.
The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:
parseFloat may interpret only a leading portion of string as aNumber value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.
19.2.5 parseInt ( string, radix )
The parseInt function produces anintegral Numberdictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix isundefinedor 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.
The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:
If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
If S is not empty and the first code unit of S is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from S.
If the length of S is at least 2 and the first two code units of S are either"0x"or"0X", then
Remove the first two code units from S.
Set R to 16.
If S contains a code unit that is not a radix-R digit, let end be the index within S of the first such code unit; otherwise, let end be the length of S.
Let mathInt be theintegervalue that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be animplementation-approximatedvalue representing theintegervalue that is represented by Z in radix-R notation.)
parseInt may interpret only a leading portion of string as anintegervalue; it ignores any code units that cannot be interpreted as part of the notation of aninteger, and no indication is given that any such code units were ignored.
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in19.2.6.2,19.2.6.3,19.2.6.4and19.2.6.5
Note
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
19.2.6.1 URI Syntax and Semantics
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme:First/Second;Third?Fourth
where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, withsurrogate pairsfirst converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0, 127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form"%xx".
19.2.6.1.1 Encode ( string, unescapedSet )
The abstract operation Encode takes arguments string (a String) and unescapedSet (a String). It performs URI encoding and escaping. It performs the following steps when called:
the String representation of octet, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
19.2.6.1.2 Decode ( string, reservedSet )
The abstract operation Decode takes arguments string (a String) and reservedSet (a String). It performs URI unescaping and decoding. It performs the following steps when called:
Let strLen be the length of string.
Let R be the empty String.
Let k be 0.
Repeat,
If k = strLen, return R.
Let C be the code unit at index k within string.
If C is not the code unit 0x0025 (PERCENT SIGN), then
Let S be the String value containing only the code unit C.
Else,
Let start be k.
If k + 2 ≥ strLen, throw aURIErrorexception.
If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw aURIErrorexception.
Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
Set k to k + 2.
Let n be the number of leading 1 bits in B.
If n = 0, then
Let C be the code unit whose value is B.
If C is not in reservedSet, then
Let S be the String value containing only the code unit C.
Else,
Let S be thesubstringof string from start to k + 1.
Else,
If n = 1 or n > 4, throw aURIErrorexception.
If k + (3 × (n - 1)) ≥ strLen, throw aURIErrorexception.
This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n > 1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified inTable 53.
Table 53 (Informative): UTF-8 Encodings
Code Unit Value
Representation
1st Octet
2nd Octet
3rd Octet
4th Octet
0x0000 - 0x007F
00000000 0zzzzzzz
0zzzzzzz
0x0080 - 0x07FF
00000yyy yyzzzzzz
110yyyyy
10zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
0xD800 - 0xDBFF followed by 0xDC00 - 0xDFFF
110110vv vvwwwwxx followed by 110111yy yyzzzzzz
11110uuu
10uuwwww
10xxyyyy
10zzzzzz
0xD800 - 0xDBFF not followed by 0xDC00 - 0xDFFF
causes URIError
0xDC00 - 0xDFFF
causes URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
Where uuuuu = vvvv + 1 to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).
The above transformation combines eachsurrogate pair(for which code unit values in the inclusive range 0xD800 to 0xDFFF are reserved) into a UTF-32 representation and encodes the resulting 21-bit value into UTF-8. Decoding reconstructs thesurrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw aURIErrorwhen encountering such invalid sequences.
19.2.6.2 decodeURI ( encodedURI )
The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:
The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.
The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:
Let componentString be ? ToString(encodedURIComponent).
The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.
The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:
The code point # is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.
19.2.6.5 encodeURIComponent ( uriComponent )
The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.
The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:
The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:
20.1.2.3 Object.defineProperties ( O, Properties )
The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:
IfType(O) is not Object, throw aTypeErrorexception.
20.1.2.4 Object.defineProperty ( O, P, Attributes )
The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:
IfType(O) is not Object, throw aTypeErrorexception.
has an [[Extensible]] internal slot whose value istrue.
has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in10.4.7.1. (Thus, it is animmutable prototype exotic object.)
has a [[Prototype]] internal slot whose value isnull.
20.1.3.1 Object.prototype.constructor
The initial value of Object.prototype.constructor is%Object%.
20.1.3.2 Object.prototype.hasOwnProperty ( V )
When the hasOwnProperty method is called with argument V, the following steps are taken:
The ordering of steps1and2is chosen to ensure that any exception that would have been thrown by step1in previous editions of this specification will continue to be thrown even if thethisvalue isundefinedornull.
20.1.3.3 Object.prototype.isPrototypeOf ( V )
When the isPrototypeOf method is called with argument V, the following steps are taken:
The ordering of steps1and2preserves the behaviour specified by previous editions of this specification for the case where V is not an object and thethisvalue isundefinedornull.
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
When the propertyIsEnumerable method is called with argument V, the following steps are taken:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps1and2is chosen to ensure that any exception that would have been thrown by step1in previous editions of this specification will continue to be thrown even if thethisvalue isundefinedornull.
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and%TypedArray%provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
20.1.3.6 Object.prototype.toString ( )
When the toString method is called, the following steps are taken:
If thethisvalue isundefined, return"[object Undefined]".
Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use@@toStringTagin ways that will invalidate the reliability of such legacy type tests.
20.1.3.7 Object.prototype.valueOf ( )
When the valueOf method is called, the following steps are taken:
Object.prototype.__proto__ is anaccessor propertywith attributes { [[Enumerable]]:false, [[Configurable]]:true}. The [[Get]] and [[Set]] attributes are defined as follows:
20.1.3.8.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:
is the initial value of the"Function"property of theglobal object.
creates and initializes a newfunction objectwhen called as a function rather than as aconstructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Functionconstructorto create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “ p ” arguments, and where body might also not be provided), the following steps are taken:
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction takes arguments constructor (aconstructor), newTarget (aconstructor), kind (eithernormal,generator,async, orasyncGenerator), and args (aListof ECMAScript language values). constructor is theconstructorfunction that is performing this action. newTarget is theconstructorthat new was initially applied to. args is the argument values that were passed to constructor. It performs the following steps when called:
If body is aListof errors, throw aSyntaxErrorexception.
NOTE: The parameters and body are parsed separately to ensure that each is valid alone. For example, new Function("/*", "*/ ) {") is not legal.
NOTE: If this step is reached, sourceText must match exprSym (although the reverse implication does not hold). The purpose of the next two steps is to enforce any Early Error rules which apply to exprSym directly.
NOTE: Functions whose kind isasyncare not constructible and do not have a [[Construct]] internal method or a"prototype"property.
Return F.
Note
CreateDynamicFunction defines a"prototype"property on any function it creates whose kind is notasyncto provide for the possibility that the function will be used as aconstructor.
has a"name"property whose value is the empty String.
Note
The Function prototype object is specified to be afunction objectto ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The thisArg value is passed without modification as thethisvalue. This is a change from Edition 3, where anundefinedornullthisArg is replaced with theglobal objectandToObjectis applied to all other values and that result is passed as thethisvalue. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or abound function exotic objectthen the thisArg will be ignored by the function [[Call]] in step6.
The thisArg value is passed without modification as thethisvalue. This is a change from Edition 3, where anundefinedornullthisArg is replaced with theglobal objectandToObjectis applied to all other values and that result is passed as thethisvalue. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or abound function exotic objectthen the thisArg will be ignored by the function [[Call]] in step4.
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is%Function%.
20.2.3.5 Function.prototype.toString ( )
When the toString method is called, the following steps are taken:
Let func be thethisvalue.
IfType(func) is Object and func has a [[SourceText]] internal slot and func.[[SourceText]] is a sequence of Unicode code points and ! HostHasSourceTextAvailable(func) istrue, then
If func is abuilt-in function object, return animplementation-definedString source code representation of func. The representation must have the syntax of aNativeFunction. Additionally, if func has an [[InitialName]] internal slot and func.[[InitialName]] is a String, the portion of the returned String that would be matched byNativeFunctionAccessoroptPropertyNamemust be the value of func.[[InitialName]].
The value of the"name"property of this function is"[Symbol.hasInstance]".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Note
This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specificconstructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
Aconstructorfunction can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.
20.2.4 Function Instances
Every Function instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. Function objects created using the Function.prototype.bind method (20.2.3.2) have the internal slots listed inTable 34.
Function instances have the following properties:
20.2.4.1 length
The value of the"length"property is anintegral Numberthat indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its"length"property depends on the function. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
20.2.4.2 name
The value of the"name"property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable orproperty namethat is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty String as the value of the"name"property.
20.2.4.3 prototype
Function instances that can be used as aconstructorhave a"prototype"property. Whenever such a Function instance is created anotherordinary objectis also created and is the initial value of the function's"prototype"property. Unless otherwise specified, the value of the"prototype"property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as aconstructor.
This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
Thehost-definedabstract operation HostHasSourceTextAvailable takes argument func (afunction object). It allowshostenvironments to prevent the source text from being provided for func.
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
It must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same completion record.
The default implementation of HostHasSourceTextAvailable is to returnNormalCompletion(true).
is the initial value of the"Boolean"property of theglobal object.
creates and initializes a new Boolean object when called as aconstructor.
performs a type conversion when called as a function rather than as aconstructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Booleanconstructorto create and initialize the subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 Boolean ( value )
When Boolean is called with argument value, the following steps are taken:
Boolean instances are ordinary objects that inherit properties from theBoolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
Append theRecord{ [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistryList.
Return newSymbol.
The GlobalSymbolRegistry is aListthat is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new emptyList. Elements of the GlobalSymbolRegistry are Records with the structure defined inTable 54.
The value of the"name"property of this function is"[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
Note
The argument is ignored.
20.4.3.6 Symbol.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"Symbol".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties from theSymbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
20.5 Error Objects
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in20.5.5or a new instance of AggregateError object defined in20.5.7. Each of these objects has the structure described below, differing only in the name used as theconstructorname instead of NativeError, in the name property of the prototype object, in theimplementation-definedmessage property of the prototype object, and in the presence of the%AggregateError%-specific errors property.
is the initial value of the"Error"property of theglobal object.
creates and initializes a new Error object when called as a function rather than as aconstructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Errorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 Error ( message )
When the Error function is called with argument message, the following steps are taken:
If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
If msg isundefined, set msg to the empty String; otherwise set msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return thestring-concatenationof name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.
20.5.4 Properties of Error Instances
Error instances are ordinary objects that inherit properties from theError prototype objectand have an [[ErrorData]] internal slot whose value isundefined. The only specified uses of [[ErrorData]] is to identify Error, AggregateError, and NativeError instances as Error objects within Object.prototype.toString.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object is thrown when a runtime error is detected. All NativeError objects share the same structure, as described in20.5.6.
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.
20.5.6NativeError Object Structure
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in20.5.5. Each of these objects has the structure described below, differing only in the name used as theconstructorname instead of NativeError, in the"name"property of the prototype object, and in theimplementation-defined"message"property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from20.5.5.
creates and initializes a new NativeError object when called as a function rather than as aconstructor. A call of the object as a function is equivalent to calling it as aconstructorwith the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeErrorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.6.1.1NativeError ( message )
When a NativeError function is called with argument message, the following steps are taken:
If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
The actual value of the string passed in step2is either"%EvalError.prototype%","%RangeError.prototype%","%ReferenceError.prototype%","%SyntaxError.prototype%","%TypeError.prototype%", or"%URIError.prototype%"corresponding to which NativeErrorconstructoris being defined.
20.5.6.2 Properties of the NativeError Constructors
The initial value of the"constructor"property of the prototype for a given NativeErrorconstructoris the corresponding intrinsic object %NativeError% (20.5.6.1).
20.5.6.3.2NativeError.prototype.message
The initial value of the"message"property of the prototype for a given NativeErrorconstructoris the empty String.
20.5.6.3.3NativeError.prototype.name
The initial value of the"name"property of the prototype for a given NativeErrorconstructoris the String value consisting of the name of theconstructor(the name used instead of NativeError).
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value isundefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.
is the initial value of the"AggregateError"property of theglobal object.
creates and initializes a new AggregateError object when called as a function rather than as aconstructor. Thus the function call AggregateError(…) is equivalent to the object creation expression new AggregateError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AggregateError behaviour must include a super call to the AggregateErrorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.7.1.1 AggregateError ( errors, message )
When theAggregateErrorfunction is called with arguments errors and message, the following steps are taken:
If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
The initial value of AggregateError.prototype.constructor is%AggregateError%.
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is"AggregateError".
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit properties from theirAggregateError prototype objectand have an [[ErrorData]] internal slot whose value isundefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.
is the initial value of the"Number"property of theglobal object.
creates and initializes a new Number object when called as aconstructor.
performs a type conversion when called as a function rather than as aconstructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Numberconstructorto create and initialize the subclass instance with a [[NumberData]] internal slot.
21.1.1.1 Number ( value )
When Number is called with argument value, the following steps are taken:
The value of Number.EPSILON is theNumber valuefor the magnitude of the difference between 1 and the smallest value greater than 1 that is representable as aNumber value, which is approximately 2.2204460492503130808472633361816 × 10-16.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.2 Number.isFinite ( number )
When Number.isFinite is called with one argument number, the following steps are taken:
This function differs from the global isNaN function (19.2.3) in that it does not convert its argument to a Number before determining whether it isNaN.
21.1.2.5 Number.isSafeInteger ( number )
When Number.isSafeInteger is called with one argument number, the following steps are taken:
The value of Number.MAX_SAFE_INTEGER is the largestintegral Numbern such thatℝ(n) andℝ(n) + 1 are both exactly representable as aNumber value.
The value of Number.MAX_SAFE_INTEGER is9007199254740991𝔽 (𝔽(253 - 1)).
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.8 Number.MIN_SAFE_INTEGER
Note
The value of Number.MIN_SAFE_INTEGER is the smallestintegral Numbern such thatℝ(n) andℝ(n) - 1 are both exactly representable as aNumber value.
The value of Number.MIN_SAFE_INTEGER is-9007199254740991𝔽 (𝔽(-(253 - 1))).
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately5 × 10-324.
In theIEEE 754-2019double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.10 Number.NaN
The value of Number.NaN isNaN.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is-∞𝔽.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and thethisvalue passed to them must be either aNumber valueor an object that has a [[NumberData]] internal slot that has been initialized to aNumber value.
The abstract operation thisNumberValue takes argument value. It performs the following steps when called:
The phrase “thisNumber value” within the specification of a method refers to the result returned by calling the abstract operationthisNumberValuewith thethisvalue of the method invocation passed as the argument.
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is%Number%.
Return a String containing thisNumber valuerepresented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits isundefined, include as many significand digits as necessary to uniquely specify the Number (just like inToStringexcept that in this case the Number is always output in exponential notation). Specifically, perform the following steps:
Let m be the String value consisting of f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
If fractionDigits is notundefined, then
Let e and n be integers such that 10f ≤ n < 10f + 1 and for which n × 10e - f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - f is larger.
Else,
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1,𝔽(n × 10e - f) is𝔽(x), and f is as small as possible. Note that the decimal representation of n has f + 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step10.b.ibe used as a guideline:
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1,𝔽(n × 10e - f) is𝔽(x), and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which𝔽(n × 10e - f) is closest in value to𝔽(x). If there are two such possible values of n, choose the one that is even.
toFixed returns a String containing thisNumber valuerepresented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits isundefined, 0 is assumed.
Let n be anintegerfor which n / 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
If n = 0, let m be the String"0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let k be the length of m.
If k ≤ f, then
Let z be the String value consisting of f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,
(1000000000000000128).toString() returns"1000000000000000100", while (1000000000000000128).toFixed(0) returns"1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents thisNumber valueformatted according to the conventions of thehost environment's current locale. This function isimplementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Return a String containing thisNumber valuerepresented either in decimal exponential notation with one digit before the significand's decimal point andprecision - 1digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision isundefined, callToStringinstead. Specifically, perform the following steps:
Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
Let e and n be integers such that 10p - 1 ≤ n < 10p and for which n × 10e - p + 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - p + 1 is larger.
Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
Set m to thestring-concatenationof the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code units of m.
Else,
Set m to thestring-concatenationof the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
Return the String representation of thisNumber valueusing the radix specified by radixMV. Letters a-z are used for digits with values 10 through 35. The precise algorithm isimplementation-defined, however the algorithm should be a generalization of that specified in6.1.6.1.20.
The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
Number instances are ordinary objects that inherit properties from theNumber prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is theNumber valuerepresented by this Number object.
is the initial value of the"BigInt"property of theglobal object.
performs a type conversion when called as a function rather than as aconstructor.
is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the BigIntconstructorwill cause an exception.
21.2.1.1 BigInt ( value )
When BigInt is called with argument value, the following steps are taken:
If NewTarget is notundefined, throw aTypeErrorexception.
The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operationthisBigIntValuewith thethisvalue of the method invocation passed as the argument.
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is%BigInt%.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the BigInt.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents this BigInt value formatted according to the conventions of thehost environment's current locale. This function isimplementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
Note
The optional radix should be anintegral Numbervalue in the inclusive range2𝔽 to36𝔽. If radix isundefinedthen10𝔽 is used as the value of radix.
Return the String representation of thisNumber valueusing the radix specified by radixMV. Letters a-z are used for digits with values 10 through 35. The precise algorithm isimplementation-defined, however the algorithm should be a generalization of that specified in6.1.6.2.23.
The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “theNumber valuefor x” has a technical meaning defined in6.1.6.1.
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
TheNumber valuefor e, the base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.3.1.2 Math.LN10
TheNumber valuefor the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.3.1.3 Math.LN2
TheNumber valuefor the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.3.1.4 Math.LOG10E
TheNumber valuefor the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.
21.3.1.5 Math.LOG2E
TheNumber valuefor the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.
21.3.1.6 Math.PI
TheNumber valuefor π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.3.1.7 Math.SQRT1_2
TheNumber valuefor the square root of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.
21.3.1.8 Math.SQRT2
TheNumber valuefor the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
21.3.1.9 Math [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"Math".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
21.3.2 Function Properties of the Math Object
Note
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms forIEEE 754-2019arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
Returns the absolute value of x; the result has the same magnitude as x but has positive sign.
When the Math.abs method is called with argument x, the following steps are taken:
Returns the inverse tangent of the quotienty / xof the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument inverse tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π, inclusive.
When the Math.atan2 method is called with arguments y and x, the following steps are taken:
Returns the result of subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close to 0.
When the Math.expm1 method is called with argument x, the following steps are taken:
If number is neither+0𝔽 nor-0𝔽, set onlyZero tofalse.
If onlyZero istrue, return+0𝔽.
Return animplementation-approximatedvalue representing the square root of the sum of squares of the mathematical values of the elements of coerced.
The"length"property of the hypot method is2𝔽.
Note
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.
21.3.2.19 Math.imul ( x, y )
When Math.imul is called with arguments x and y, the following steps are taken:
Returns aNumber valuewith positive sign, greater than or equal to+0𝔽 but strictly less than1𝔽, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using animplementation-definedalgorithm or strategy. This function takes no arguments.
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
21.3.2.28 Math.round ( x )
Returns theNumber valuethat is closest to x and is integral. If two integral Numbers are equally close to x, then the result is theNumber valuethat is closer to +∞. If x is already integral, the result is x.
When the Math.round method is called with argument x, the following steps are taken:
Return theintegral Numberclosest to n, preferring the Number closer to +∞ in the case of a tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of Math.floor(x + 0.5). When x is-0𝔽 or is less than+0𝔽 but greater than or equal to-0.5𝔽, Math.round(x) returns-0𝔽, but Math.floor(x + 0.5) returns+0𝔽. Math.round(x) may also differ from the value of Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.
21.3.2.29 Math.sign ( x )
Returns the sign of x, indicating whether x is positive, negative, or zero.
When the Math.sign method is called with argument x, the following steps are taken:
21.4.1 Overview of Date Objects and Definitions of Abstract Operations
The followingabstract operationsoperate on time values (defined in21.4.1.1). Note that, in every case, if any argument to one of these functions isNaN, the result will beNaN.
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a Number, either a finiteintegral Numberrepresenting an instant in time to millisecond precision orNaNrepresenting no specific instant. A time value that is a multiple of24 × 60 × 60 × 1000 = 86,400,000(i.e., is equal to 86,400,000 × d for someintegerd) represents the instant at the start of the UTC day that follows the epoch by d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to the greatest preceding time value s that is such a multiple, and represents the instant that occurs within the same UTC day as s but follows it by t − s milliseconds.
Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (21.1.2.8and21.1.2.6). A time value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 1 January 1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value+0𝔽.
Note
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.
ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:
Months are identified by anintegral Numberin the range+0𝔽 to11𝔽, inclusive. The mappingMonthFromTime(t) from atime valuet to a month number is defined by:
A month value of+0𝔽 specifies January;1𝔽 specifies February;2𝔽 specifies March;3𝔽 specifies April;4𝔽 specifies May;5𝔽 specifies June;6𝔽 specifies July;7𝔽 specifies August;8𝔽 specifies September;9𝔽 specifies October;10𝔽 specifies November; and11𝔽 specifies December. Note thatMonthFromTime(+0𝔽) =+0𝔽, corresponding to Thursday, 1 January 1970.
21.4.1.5 Date Number
A date number is identified by anintegral Numberin the range1𝔽 through31𝔽, inclusive. The mapping DateFromTime(t) from atime valuet to a date number is defined by:
A weekday value of+0𝔽 specifies Sunday;1𝔽 specifies Monday;2𝔽 specifies Tuesday;3𝔽 specifies Wednesday;4𝔽 specifies Thursday;5𝔽 specifies Friday; and6𝔽 specifies Saturday. Note thatWeekDay(+0𝔽) =4𝔽, corresponding to Thursday, 1 January 1970.
21.4.1.7 LocalTZA ( t, isUTC )
Theimplementation-definedabstract operation LocalTZA takes arguments t and isUTC. It returns anintegral Numberrepresenting the local time zone adjustment, or offset, in milliseconds. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section.
When isUTC is true,LocalTZA( tUTC, true )should return the offset of the local time zone from UTC measured in milliseconds at time represented bytime valuetUTC. When the result is added totUTC, it should yield the corresponding Numbertlocal.
When isUTC is false,LocalTZA( tlocal, false )should return the offset of the local time zone from UTC measured in milliseconds at local time represented by Numbertlocal. When the result is subtracted fromtlocal, it should yield the correspondingtime valuetUTC.
Input t is nominally atime valuebut may be anyNumber value. This can occur when isUTC is false and tlocal represents atime valuethat is already offset outside of thetime valuerange at the range boundaries. The algorithm must not limit tlocal to thetime valuerange, so that such inputs are supported.
Whentlocalrepresents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or skipped local time at a positive time zone transitions (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change),tlocalmust be interpreted using the time zone offset before the transition.
If an implementation does not support a conversion described above or if political rules for time t are not available within the implementation, the result must be+0𝔽.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. LocalTZA(TimeClip(MakeDate(MakeDay(2017, 10, 5),MakeTime(1, 30, 0, 0))), false) is-4 ×msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). LocalTZA(TimeClip(MakeDate(MakeDay(2017, 2, 12),MakeTime(2, 30, 0, 0))), false) is-5 ×msPerHour.
Local time zone offset values may be positive or negative.
21.4.1.8 LocalTime ( t )
The abstract operation LocalTime takes argument t. It converts t from UTC to local time. It performs the following steps when called:
Two different input time valuestUTCare converted to the same local timetlocalat a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).
LocalTime(UTC(tlocal))is not necessarily always equal totlocal. Correspondingly,UTC(LocalTime(tUTC))is not necessarily always equal totUTC.
21.4.1.9 UTC ( t )
The abstract operation UTC takes argument t. It converts t from local time to UTC. It performs the following steps when called:
The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number). It calculates a number of milliseconds. It performs the following steps when called:
If hour is not finite or min is not finite or sec is not finite or ms is not finite, returnNaN.
The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number). It calculates a number of days. It performs the following steps when called:
If year is not finite or month is not finite or date is not finite, returnNaN.
The abstract operation MakeDate takes arguments day (a Number) and time (a Number). It calculates a number of milliseconds. It performs the following steps when called:
If day is not finite or time is not finite, returnNaN.
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as anexpanded yearof"+"or"-"followed by six decimal digits.
-
"-"(hyphen) appears literally twice in the string.
MM
is the month of the year as two decimal digits from 01 (January) to 12 (December).
DD
is the day of the month as two decimal digits from 01 to 31.
T
"T"appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:
":"(colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
.
"."(dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the UTC offset representation specified as"Z"(for UTC with no offset) or an offset of either"+"or"-"followed by a time expression HH:mm (indicating local time ahead of or behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.
21.4.1.15.1 Expanded Years
Coveringthe fulltime valuerange of approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1) requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual agreement of the partners in information interchange. In the simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign. Strings matching theDate Time String Formatwith expanded years representing instants in time outside the range of atime valueare treated as unrecognizable byDate.parseand cause that function to returnNaNwithout falling back to implementation-specific behaviour or heuristics.
is the initial value of the"Date"property of theglobal object.
creates and initializes a new Date object when called as aconstructor.
returns a String representing the current time (UTC) when called as a function rather than as aconstructor.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Dateconstructorto create and initialize the subclass instance with a [[DateValue]] internal slot.
has a"length"property whose value is7𝔽.
21.4.2.1 Date ( ...values )
When the Date function is called, the following steps are taken:
If NewTarget isundefined, then
Let now be thetime value(UTC) identifying the current time.
The now function returns thetime valuedesignating the UTC date and time of the occurrence of the call to now.
21.4.3.2 Date.parse ( string )
The parse function applies theToStringoperator to its argument. IfToStringresults in anabrupt completiontheCompletion Recordis immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTCtime valuecorresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the String according to the format described in Date Time String Format (21.4.1.15), including expanded years. If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format element values shall cause Date.parse to returnNaN.
If the String conforms to theDate Time String Format, substitute values take the place of absent format elements. When the MM or DD elements are absent,"01"is used. When the HH, mm, or ss elements are absent,"00"is used. When the sss element is absent,"000"is used. When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
is not required to produce the sameNumber valueas the preceding three expressions and, in general, the value produced by Date.parse isimplementation-definedwhen given any String value that does not conform to the Date Time String Format (21.4.1.15) and that could not be produced in that implementation by the toString or toUTCString method.
The UTC function differs from the Dateconstructorin two ways: it returns atime valueas a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and thethisvalue passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to atime value.
The abstract operation thisTimeValue takes argument value. It performs the following steps when called:
IfType(value) is Object and value has a [[DateValue]] internal slot, then
Return value.[[DateValue]].
Throw aTypeErrorexception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is thethisvalue for the invocation of the function. If the Type of thethisvalue is not Object, aTypeErrorexception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operationthisTimeValuewith thethisvalue of the method invocation passed as the argument.
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is%Date%.
The"length"property of the setFullYear method is3𝔽.
Note
If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )
If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().
The"length"property of the setMinutes method is3𝔽.
Note
If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
The"length"property of the setUTCFullYear method is3𝔽.
Note
If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )
The"length"property of the setUTCHours method is4𝔽.
Note
If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
The"length"property of the setUTCMinutes method is3𝔽.
Note
If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it function behaves as if ms was present with the value return by getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )
Ifthis time valueis not a finite Number or if it corresponds with a year that cannot be represented in theDate Time String Format, this function throws aRangeErrorexception. Otherwise, it returns a String representation ofthis time valuein that format on the UTC time scale, including all format elements and the UTC offset representation"Z".
21.4.4.37 Date.prototype.toJSON ( key )
This function provides a String representation of a Date object for use by JSON.stringify (25.5.2).
When the toJSON method is called with argument key, the following steps are taken:
The toJSON function is intentionally generic; it does not require that itsthisvalue be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.
This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.
This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
For any Date object d such that d.[[DateValue]] is evenly divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See21.4.3.2.
Note 2
The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv. It performs the following steps when called:
Let hour be the String representation ofHourFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Let minute be the String representation ofMinFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Let second be the String representation ofSecFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Return thestring-concatenationof hour,":", minute,":", second, the code unit 0x0020 (SPACE), and"GMT".
21.4.4.41.2 DateString ( tv )
The abstract operation DateString takes argument tv. It performs the following steps when called:
Let day be the String representation ofDateFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Return thestring-concatenationof weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
Table 55: Names of days of the week
Number
Name
+0𝔽
"Sun"
1𝔽
"Mon"
2𝔽
"Tue"
3𝔽
"Wed"
4𝔽
"Thu"
5𝔽
"Fri"
6𝔽
"Sat"
Table 56: Names of months of the year
Number
Name
+0𝔽
"Jan"
1𝔽
"Feb"
2𝔽
"Mar"
3𝔽
"Apr"
4𝔽
"May"
5𝔽
"Jun"
6𝔽
"Jul"
7𝔽
"Aug"
8𝔽
"Sep"
9𝔽
"Oct"
10𝔽
"Nov"
11𝔽
"Dec"
21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv. It performs the following steps when called:
Let offsetMin be the String representation ofMinFromTime(absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Let offsetHour be the String representation ofHourFromTime(absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Let tzName be animplementation-definedstring that is either the empty String or thestring-concatenationof the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), animplementation-definedtimezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
The toUTCString method returns a String value representing the instance in time corresponding tothis time value. The format of the String is based upon "HTTP-date" from RFC 7231, generalized to support the full range of times supported by ECMAScript Date objects. It performs the following steps when called:
Let day be the String representation ofDateFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
Return thestring-concatenationof weekday,",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), andTimeString(tv).
21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are"default","number", and"string". Date objects, are unique among built-in ECMAScript object in that they treat"default"as being equivalent to"string", All other built-in ECMAScript objects treat"default"as being equivalent to"number".
When the @@toPrimitive method is called with argument hint, the following steps are taken:
Let O be thethisvalue.
IfType(O) is not Object, throw aTypeErrorexception.
The value of the"name"property of this function is"[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
21.4.5 Properties of Date Instances
Date instances are ordinary objects that inherit properties from theDate prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is thetime valuerepresented bythis Date object.
is the initial value of the"String"property of theglobal object.
creates and initializes a new String object when called as aconstructor.
performs a type conversion when called as a function rather than as aconstructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the Stringconstructorto create and initialize the subclass instance with a [[StringData]] internal slot.
22.1.1.1 String ( value )
When String is called with argument value, the following steps are taken:
If value is not present, let s be the empty String.
The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form theListsubstitutions. The following steps are taken:
Let numberOfSubstitutions be the number of elements in substitutions.
Append the code unit elements of nextSub to the end of stringElements.
Set nextIndex to nextIndex + 1.
Note
The raw function is intended for use as a tag function of a Tagged Template (13.3.11). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.
22.1.3 Properties of the String Prototype Object
The String prototype object:
is %String.prototype%.
is aString exotic objectand has the internal methods specified for such objects.
has a [[StringData]] internal slot whose value is the empty String.
has a"length"property whose initial value is+0𝔽 and whose attributes are { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and thethisvalue passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.
The abstract operation thisStringValue takes argument value. It performs the following steps when called:
Returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.
If pos is anintegral Number, then the result of x.charAt(pos) is equivalent to the result of x.substring(pos, pos + 1).
When the charAt method is called with one argument pos, the following steps are taken:
If position < 0 or position ≥ size, return the empty String.
Return the String value of length 1, containing one code unit from S, namely the code unit at index position.
Note 2
The charAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.2 String.prototype.charCodeAt ( pos )
Note 1
Returns a Number (a non-negativeintegral Numberless than 216) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result isNaN.
When the charCodeAt method is called with one argument pos, the following steps are taken:
Return theNumber valuefor the numeric value of the code unit at index position within the String S.
Note 2
The charCodeAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.3 String.prototype.codePointAt ( pos )
Note 1
Returns a non-negativeintegral Numberless than or equal to0x10FFFF𝔽 that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result isundefined. If a valid UTF-16surrogate pairdoes not begin at pos, the result is the code unit at pos.
When the codePointAt method is called with one argument pos, the following steps are taken:
The codePointAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.4 String.prototype.concat ( ...args )
Note 1
When the concat method is called it returns the String value consisting of the code units of thethisvalue (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.
When the concat method is called with zero or more arguments, the following steps are taken:
The concat function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.5 String.prototype.constructor
The initial value of String.prototype.constructor is%String%.
Returnstrueif the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at endPosition - length(this). Otherwise returnsfalse.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The endsWith function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.7 String.prototype.includes ( searchString [ , position ] )
The includes method takes two arguments, searchString and position, and performs the following steps:
If searchString appears as asubstringof the result of converting this object to a String, at one or more indices that are greater than or equal to position, returntrue; otherwise, returnsfalse. If position isundefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The includes function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.8 String.prototype.indexOf ( searchString [ , position ] )
Note 1
If searchString appears as asubstringof the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise,-1𝔽 is returned. If position isundefined,+0𝔽 is assumed, so as to search all of the String.
The indexOf method takes two arguments, searchString and position, and performs the following steps:
The indexOf function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.9 String.prototype.lastIndexOf ( searchString [ , position ] )
Note 1
If searchString appears as asubstringof the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise,-1𝔽 is returned. If position isundefined, the length of the String value is assumed, so as to search all of the String.
The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:
Assert: If position isundefined, then numPos isNaN.
If numPos isNaN, let pos be +∞; otherwise, let pos be ! ToIntegerOrInfinity(numPos).
Let len be the length of S.
Let start be the result ofclampingpos between 0 and len.
Let searchLen be the length of searchStr.
Let k be the largest possible non-negativeintegernot larger than start such that k + searchLen ≤ len, and for all non-negative integers j such that j < searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr; but if there is no suchinteger, let k be -1.
The lastIndexOf function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.
When the localeCompare method is called with argument that, it returns a Number other thanNaNthat represents the result of a locale-sensitive String comparison of thethisvalue (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in animplementation-definedfashion. The result is intended to order String values in the sort order specified by ahostdefault locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.
Before performing the comparisons, the following steps are performed to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The localeCompare method, if considered as a function of two argumentsthisand that, is a consistent comparison function (as defined in23.1.3.27) on the set of all Strings.
The actual return values areimplementation-definedto permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.
Note 1
The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from thehost environment, and to compare according to the rules of thehost environment's current locale. However, regardless of thehostprovided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (https://unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (https://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (https://unicode.org/reports/tr10/).
Note 3
The localeCompare function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.11 String.prototype.match ( regexp )
When the match method is called with argument regexp, the following steps are taken:
The match function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.12 String.prototype.matchAll ( regexp )
Performs a regular expression match of the String representing thethisvalue against regexp and returns an iterator. Each iteration result's value is an Array object containing the results of the match, ornullif the String did not match.
When the matchAll method is called, the following steps are taken:
The matchAll function is intentionally generic, it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Note 2
Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.
22.1.3.13 String.prototype.normalize ( [ form ] )
When the normalize method is called with one argument form, the following steps are taken:
If f is not one of"NFC","NFD","NFKC", or"NFKD", throw aRangeErrorexception.
Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
Return ns.
Note
The normalize function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Return the String value that is made from n copies of S appended together.
Note 1
This method creates the String value consisting of the code units of thethisvalue (converted to String) repeated count times.
Note 2
The repeat function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The replace function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The abstract operation GetSubstitution takes arguments matched, str, position (a non-negativeinteger), captures, namedCaptures, and replacement. It performs the following steps when called:
Let result be the String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified inTable 57. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
Return result.
Table 57: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
The replacement is thesubstringof str from 0 to position.
0x0024, 0x0027
$'
If tailPos ≥ stringLength, the replacement is the empty String. Otherwise the replacement is thesubstringof str from tailPos.
0x0024, N Where 0x0031 ≤ N ≤ 0x0039
$n where n is one of 1 2 3 4 5 6 7 8 9 and $n is not followed by a decimal digit
The nth element of captures, where n is a single digit in the range 1 to 9. If n ≤ m and the nth element of captures isundefined, use the empty String instead. If n > m, no replacement is done.
0x0024, N, N Where 0x0030 ≤ N ≤ 0x0039
$nn where n is one of 0 1 2 3 4 5 6 7 8 9
The nnth element of captures, where nn is a two-digit decimal number in the range 01 to 99. If nn ≤ m and the nnth element of captures isundefined, use the empty String instead. If nn is 00 or nn > m, no replacement is done.
0x0024, 0x003C
$<
If namedCaptures isundefined, the replacement text is the String"$<".
The search function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.20 String.prototype.slice ( start, end )
The slice method takes two arguments, start and end, and returns asubstringof the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end isundefined). If start is negative, it is treated assourceLength + startwhere sourceLength is the length of the String. If end is negative, it is treated assourceLength + endwhere sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
The slice function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any String in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a@@splitmethod.
When the split method is called, the following steps are taken:
The value of separator may be an empty String. In this case, separator does not match the emptysubstringat the beginning or end of the input String, nor does it match the emptysubstringat the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and eachsubstringcontains one code unit.
If thethisvalue is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If separator isundefined, then the result array contains just one String, which is thethisvalue (converted to a String). If limit is notundefined, then the output array is truncated so that it contains no more than limit elements.
Note 2
The split function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.21.1 SplitMatch ( S, q, R )
The abstract operation SplitMatch takes arguments S (a String), q (a non-negativeinteger), and R (a String). It returns eithernot-matchedor the end index of a match. It performs the following steps when called:
Let r be the number of code units in R.
Let s be the number of code units in S.
If q + r > s, returnnot-matched.
If there exists anintegeri between 0 (inclusive) and r (exclusive) such that the code unit at index q + i within S is different from the code unit at index i within R, returnnot-matched.
Return q + r.
22.1.3.22 String.prototype.startsWith ( searchString [ , position ] )
This method returnstrueif the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index position. Otherwise returnsfalse.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The startsWith function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.23 String.prototype.substring ( start, end )
The substring method takes two arguments, start and end, and returns asubstringof the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end isundefined). The result is a String value, not a String object.
If either argument isNaNor negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.
The substring function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for thehost environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleLowerCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for thehost environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleUpperCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.26 String.prototype.toLowerCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4. The following steps are taken:
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().
Note 2
The toLowerCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.27 String.prototype.toString ( )
When the toString method is called, the following steps are taken:
For a String object, the toString method happens to return the same thing as the valueOf method.
22.1.3.28 String.prototype.toUpperCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
The toUpperCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.29 String.prototype.trim ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
The trim function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.29.1 TrimString ( string, where )
The abstract operation TrimString takes arguments string and where. It interprets string as a sequence of UTF-16 encoded code points, as described in6.1.4. It performs the following steps when called:
Let T be the String value that is a copy of S with both leading and trailing white space removed.
Return T.
The definition of white space is the union ofWhiteSpaceandLineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in6.1.4.
22.1.3.30 String.prototype.trimEnd ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
The trimEnd function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.31 String.prototype.trimStart ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.
The trimStart function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.32 String.prototype.valueOf ( )
When the valueOf method is called, the following steps are taken:
When the @@iterator method is called it returns an Iterator object (27.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:
The value of the"name"property of this function is"[Symbol.iterator]".
22.1.4 Properties of String Instances
String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from theString prototype object. String instances also have a [[StringData]] internal slot.
String instances have a"length"property, and a set of enumerable properties withinteger-indexed names.
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
22.1.5 String Iterator Objects
A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a namedconstructorfor String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.
22.1.5.1 The %StringIteratorPrototype% Object
The %StringIteratorPrototype% object:
has properties that are inherited by all String Iterator Objects.
The initial value of the@@toStringTagproperty is the String value"String Iterator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.
22.2.1 Patterns
The RegExpconstructorapplies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion ofPattern.
It is a Syntax Error if theListof Unicode code points that isSourceTextofUnicodePropertyValueis not identical to aListof Unicode code points that is a value or value alias for the Unicode property or property alias given bySourceTextofUnicodePropertyNamelisted in the “Property value and aliases” column of the corresponding tablesTable 61orTable 62.
It is a Syntax Error if theListof Unicode code points that isSourceTextofLoneUnicodePropertyNameOrValueis not identical to aListof Unicode code points that is a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 61, nor a binary property or binary property alias listed in the “Property nameand aliases” column ofTable 60.
A regular expression pattern is converted into anAbstract Closureusing the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. TheAbstract Closureis used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
APatternis either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a u. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics ofPatternis defined as if the source code for thePatternwas aListofSourceCharactervalues where eachSourceCharactercorresponds to a Unicode code point. If a BMP pattern contains a non-BMPSourceCharacterthe entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of theList.
Note 2
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character)Listconsisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two elementListconsisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExpconstructoras ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode patternUTF16SurrogatePairToCodePointmust be used in producing aListwhose sole element is a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.
22.2.2.1 Notation
The descriptions below use the following aliases:
Input is aListwhose elements are the characters of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the nth character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in Input.
NcapturingParens is the total number of left-capturing parentheses (i.e. the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes) in the pattern. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of theAtom::(GroupSpecifierDisjunction)production.
WordCharacters is the mathematical set that is the union of all sixty-three characters in"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_"(letters, numbers, and U+005F (LOW LINE) in the Unicode Basic Latin block) and all characters c for which c is not in that set butCanonicalize(c) is. WordCharacters cannot contain more than sixty-three characters unless Unicode and IgnoreCase are bothtrue.
Furthermore, the descriptions below use the following internal data structures:
A CharSet is a mathematical set of characters. When the Unicode flag istrue, “all characters” means the CharSet containing all code point values; otherwise “all characters” means the CharSet containing all code unit values.
A State is an ordered pair (endIndex, captures) where endIndex is anintegerand captures is aListof NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either aListof characters that represents the value obtained by the nth set of capturing parentheses orundefinedif the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special tokenfailurethat indicates that the match failed.
A Continuation is anAbstract Closurethat takes one State argument and returns a MatchResult result. The Continuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returnsfailure.
A Matcher is anAbstract Closurethat takes two arguments—a State and a Continuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
Assert: index is a non-negativeintegerwhich is ≤ the length of str.
If Unicode istrue, let Input be ! StringToCodePoints(str). Otherwise, let Input be aListwhose elements are the code units that are the elements of str. Input will be used throughout the algorithms in22.2.2. Each element of Input is considered to be a character.
Let InputLength be the number of characters contained in Input. This alias will be used throughout the algorithms in22.2.2.
Let listIndex be the index into Input of the character that was obtained from element index of str.
Let c be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
Let cap be aListof NcapturingParensundefinedvalues, indexed 1 through NcapturingParens.
Let x be the State (listIndex, cap).
Return m(x, c).
Note
A Pattern evaluates (“compiles”) to anAbstract Closurevalue.RegExpBuiltinExeccan then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in22.2.2are designed so that compiling a pattern may throw aSyntaxErrorexception; on the other hand, once the pattern is successfully compiled, applying the resultingAbstract Closureto find a match in a String cannot throw an exception (except for anyimplementation-definedexceptions that can occur anywhere such as out-of-memory).
The | regular expression operator separates two alternatives. The pattern first tries to match the leftAlternative(followed by the sequel of the regular expression); if it fails, it tries to match the rightDisjunction(followed by the sequel of the regular expression). If the leftAlternative, the rightDisjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the leftAlternative. If choices in the leftAlternativeare exhausted, the rightDisjunctionis tried instead of the leftAlternative. Any capturing parentheses inside a portion of the pattern skipped by | produceundefinedvalues instead of Strings. Thus, for example,
ConsecutiveTerms try to simultaneously match consecutive portions of Input. When direction = 1, if the leftAlternative, the rightTerm, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the rightTerm, and all choices in the rightTermare tried before moving on to the next choice in the leftAlternative. When direction = -1, the evaluation order ofAlternativeandTermare reversed.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of thisTerm. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing thisTerm.
Let parenCount be the number of left-capturing parentheses inAtom. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes enclosed byAtom.
Return a new Matcher with parameters (x, c) that captures m, min, max, greedy, parenIndex, and parenCount and performs the following steps when called:
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negativeinteger), max (a non-negativeintegeror +∞), greedy (a Boolean), x (a State), c (a Continuation), parenIndex (a non-negativeinteger), and parenCount (a non-negativeinteger). It performs the following steps when called:
If max = 0, return c(x).
Let d be a new Continuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
For eachintegerk such that parenIndex < k and k ≤ parenIndex + parenCount, set cap[k] toundefined.
Let e be x's endIndex.
Let xr be the State (e, cap).
If min ≠ 0, return m(xr, d).
If greedy isfalse, then
Let z be c(x).
If z is notfailure, return z.
Return m(xr, d).
Let z be m(xr, d).
If z is notfailure, return z.
Return c(x).
Note 1
AnAtomfollowed by aQuantifieris repeated the number of times specified by theQuantifier. AQuantifiercan be non-greedy, in which case theAtompattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case theAtompattern is repeated as many times as possible while still matching the sequel. TheAtompattern is repeated rather than the input character sequence that it matches, so different repetitions of theAtomcan match different input substrings.
Note 2
If theAtomand the sequel of the regular expression all have choice points, theAtomis first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition ofAtom. All choices in the last (nth) repetition ofAtomare tried before moving on to the next choice in the next-to-last (n - 1)st repetition ofAtom; at which point it may turn out that more or fewer repetitions ofAtomare now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n - 1)st repetition ofAtomand so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns"abcde"with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns"abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
Step4of the RepeatMatcher clearsAtom's captures each timeAtomis repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the quantifiedAtom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step2.bof the RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more expansions ofAtomthat match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:
EvaluateDisjunctionwith argument direction to obtain a Matcher m.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of thisAtom. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing thisAtom.
Return a new Matcher with parameters (x, c) that captures direction, m, and parenIndex and performs the following steps when called:
Return the Matcher that is the result of evaluatingDisjunctionwith argument direction.
22.2.2.8.1 CharacterSetMatcher ( A, invert, direction )
The abstract operation CharacterSetMatcher takes arguments A (a CharSet), invert (a Boolean), and direction (1 or -1). It performs the following steps when called:
Return a new Matcher with parameters (x, c) that captures A, invert, and direction and performs the following steps when called:
The abstract operation Canonicalize takes argument ch (a character). It performs the following steps when called:
If Unicode istrueand IgnoreCase istrue, then
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
If uStr does not consist of a single code unit, return ch.
Let cu be uStr's single code unit element.
If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
Return cu.
Note 1
Parentheses of the form (Disjunction) serve both to group the components of theDisjunctionpattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a non-zero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matchingAbstract Closure. To inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.
Note 2
The form (?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern insideDisjunctionmust match at the current position, but the current position is not advanced before matching the sequel. IfDisjunctioncan match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when theDisjunctioncontains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Note 3
The form (?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern insideDisjunctionmust fail to match at the current position. The current position is not advanced before matching the sequel.Disjunctioncan contain capturing parentheses, but backreferences to them only make sense from withinDisjunctionitself. Backreferences to these capturing parentheses from elsewhere in the pattern always returnundefinedbecause the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches againstundefinedand therefore always succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Note 4
In case-insignificant matches when Unicode istrue, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, ß (U+00DF) to SS. It may however map a code point outside the Basic Latin range to a character within, for example, ſ (U+017F) to s. Such characters are not mapped if Unicode isfalse. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.
22.2.2.8.3 UnicodeMatchProperty ( p )
The abstract operation UnicodeMatchProperty takes argument p (aListof Unicode code points). It performs the following steps when called:
Assert: p is aListof Unicode code points that is identical to aListof Unicode code points that is a Unicodeproperty nameor property alias listed in the “Property nameand aliases” column ofTable 59orTable 60.
Let c be the canonicalproperty nameof p as given in the “Canonicalproperty name” column of the corresponding row.
Implementations must support the Unicode property names and aliases listed inTable 59andTable 60. To ensure interoperability, implementations must not support any other property names or aliases.
Note 1
For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Table 59: Non-binary Unicode property aliases and their canonical property names
The abstract operation UnicodeMatchPropertyValue takes arguments p (aListof Unicode code points) and v (aListof Unicode code points). It performs the following steps when called:
Assert: p is aListof Unicode code points that is identical to aListof Unicode code points that is a canonical, unaliased Unicodeproperty namelisted in the “Canonicalproperty name” column ofTable 59.
Assert: v is aListof Unicode code points that is identical to aListof Unicode code points that is a property value or property value alias for Unicode property p listed in the “Property value and aliases” column ofTable 61orTable 62.
Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
Implementations must support the Unicode property value names and aliases listed inTable 61andTable 62. To ensure interoperability, implementations must not support any other property value names or aliases.
Note 1
For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.
An escape sequence of the form \ followed by a non-zero decimal number n matches the result of the nth set of capturing parentheses (22.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one isundefinedbecause it has not captured anything, then the backreference always succeeds.
Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of the locatedGroupSpecifier. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing the locatedGroupSpecifier, including its immediately enclosingAtom.
The abstract operation BackreferenceMatcher takes arguments n (a positiveinteger) and direction (1 or -1). It performs the following steps when called:
If there exists anintegeri between 0 (inclusive) and len (exclusive) such thatCanonicalize(s[i]) is not the same character value asCanonicalize(Input[g + i]), returnfailure.
If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left-capturing parentheses in the entire regular expression.
If ! UnicodeMatchPropertyValue(General_Category, s) is identical to aListof Unicode code points that is the name of a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 61, then
Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
ClassRangescan expand into a singleClassAtomand/or ranges of twoClassAtomseparated by dashes. In the latter case theClassRangesincludes all characters between the firstClassAtomand the secondClassAtom, inclusive; an error occurs if eitherClassAtomdoes not represent a single character (for example, if one is \w) or if the firstClassAtom's character value is greater than the secondClassAtom's character value.
Note 2
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.
Note 3
A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character ofClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.
AClassAtomcan use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside aCharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside aClassAtomcauses an error.
is the initial value of the"RegExp"property of theglobal object.
creates and initializes a new RegExp object when called as a function rather than as aconstructor. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExpconstructorto create and initialize subclass instances with the necessary internal slots.
If pattern is supplied using aStringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within theStringLiteralto prevent them being removed when the contents of theStringLiteralare formed.
22.2.3.2 Abstract Operations for the RegExp Constructor
22.2.3.2.1 RegExpAlloc ( newTarget )
The abstract operation RegExpAlloc takes argument newTarget. It performs the following steps when called:
Let obj be ? OrdinaryCreateFromConstructor(newTarget,"%RegExp.prototype%", « [[RegExpMatcher]], [[OriginalSource]], [[OriginalFlags]] »).
Set obj.[[RegExpMatcher]] to theAbstract Closurethat evaluates parseResult by applying the semantics provided in22.2.2using patternCharacters as the pattern'sListofSourceCharactervalues and F as the flag parameters.
22.2.3.2.3 Static Semantics: ParsePattern ( patternText, u )
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points) and u (a Boolean). It performs the following steps when called:
The abstract operation EscapeRegExpPattern takes arguments P and F. It performs the following steps when called:
Let S be a String in the form of aPattern[~U](Pattern[+U]if F contains"u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not be identical to P; however, theAbstract Closurethat would result from evaluating S as aPattern[~U](Pattern[+U]if F contains"u") must behave identically to theAbstract Closuregiven by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
The code points / or anyLineTerminatoroccurring in the pattern shall be escaped in S as necessary to ensure that thestring-concatenationof"/", S,"/", and F can be parsed (in an appropriate lexical context) as aRegularExpressionLiteralthat behaves identically to the constructed regular expression. For example, if P is"/", then S could be"\/"or"\u002F", among other possibilities, but not"/", because /// followed by F would be parsed as aSingleLineCommentrather than aRegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be"(?:)".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
22.2.4.2 get RegExp [ @@species ]
RegExp[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
RegExp prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.
The RegExp prototype object does not have a"valueOf"property of its own; however, it inherits the"valueOf"property from theObject prototype object.
22.2.5.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is%RegExp%.
22.2.5.2 RegExp.prototype.exec ( string )
Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, ornullif string did not match.
The StringToString(string) is searched for an occurrence of the regular expression pattern as follows:
If a callable"exec"property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of"exec".
22.2.5.2.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R and S. It performs the following steps when called:
e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the number of elements in Input, then eUTF is the number of code units in S.
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a non-negativeinteger), and unicode (a Boolean). It performs the following steps when called:
The abstract operation RegExpHasFlag takes arguments R (anECMAScript language value) and codeUnit (a code unit). It performs the following steps when called:
IfType(R) is not Object, throw aTypeErrorexception.
If R does not have an [[OriginalFlags]] internal slot, then
The value of the"name"property of this function is"[Symbol.match]".
Note
The@@matchproperty is used by theIsRegExpabstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a@@matchproperty or the existence of such a property whose value does not Boolean coerce totrueindicates that the object is not intended to be used as a regular expression object.
Let replacement be ? GetSubstitution(matched, S, position, captures, namedCaptures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
Set accumulatedResult to thestring-concatenationof accumulatedResult, thesubstringof S from nextSourcePosition to position, and replacement.
Set nextSourcePosition to position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of thethisvalue regular expression; these occurrences are not part of any String in the returned array, but serve to divide up the String value.
Thethisvalue may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the emptysubstringat the beginning or end of the input String, nor does it match the emptysubstringat the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and eachsubstringcontains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-emptysubstringmatch at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is matched the results (including anyundefinedresults) of the capturing parentheses are spliced into the output array. For example,
RegExp instances are ordinary objects that inherit properties from theRegExp prototype object. RegExp instances have internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal slot is anAbstract Closurerepresentation of thePatternof the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties"source","global","ignoreCase", and"multiline". Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
22.2.6.1 lastIndex
The value of the"lastIndex"property specifies the String index at which to start the next match. It is coerced to anintegral Numberwhen used (see22.2.5.2.2). This property shall have the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
22.2.7 RegExp String Iterator Objects
A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a namedconstructorfor RegExp String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance objects.
The initial value of the@@toStringTagproperty is the String value"RegExp String Iterator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
23 Indexed Collections
23.1 Array Objects
Array objects are exotic objects that give special treatment to a certain class of property names. See10.4.2for a definition of this special treatment.
also creates and initializes a new Array object when called as a function rather than as aconstructor. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Arrayconstructorto initialize subclass instances that are Array exotic objects. However, most of the Array.prototype methods are generic methods that are not dependent upon theirthisvalue being anArray exotic object.
has a"length"property whose value is1𝔽.
23.1.1.1 Array ( ...values )
When the Array function is called, the following steps are taken:
If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
The from function is an intentionally generic factory method; it does not require that itsthisvalue be the Arrayconstructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.
23.1.2.2 Array.isArray ( arg )
When the isArray method is called, the following steps are taken:
The of function is an intentionally generic factory method; it does not require that itsthisvalue be the Arrayconstructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
23.1.2.5 get Array [ @@species ]
Array[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
Array prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
is %Array.prototype%.
is anArray exotic objectand has the internal methods specified for such objects.
has a"length"property whose initial value is+0𝔽 and whose attributes are { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
The Array prototype object is specified to be anArray exotic objectto ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
23.1.3.1 Array.prototype.concat ( ...items )
Returns an array containing the array elements of the object followed by the array elements of each argument.
When the concat method is called, the following steps are taken:
The explicit setting of the"length"property in step6is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.
Note 2
The concat function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.1.1 IsConcatSpreadable ( O )
The abstract operation IsConcatSpreadable takes argument O. It performs the following steps when called:
The initial value of Array.prototype.constructor is%Array%.
23.1.3.3 Array.prototype.copyWithin ( target, start [ , end ] )
Note 1
The end argument is optional. If it is not provided, the length of thethisvalue is used.
Note 2
If target is negative, it is treated aslength + targetwhere length is the length of the array. If start is negative, it is treated aslength + start. If end is negative, it is treated aslength + end.
When the copyWithin method is called, the following steps are taken:
The copyWithin function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.4 Array.prototype.entries ( )
When the entries method is called, the following steps are taken:
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returnsfalse. If such an element is found, every immediately returnsfalse. Otherwise, if callbackfn returnedtruefor all elements, every will returntrue. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returnstrue.
When the every method is called, the following steps are taken:
Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
If testResult isfalse, returnfalse.
Set k to k + 1.
Returntrue.
Note 2
The every function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.6 Array.prototype.fill ( value [ , start [ , end ] ] )
Note 1
The start argument is optional. If it is not provided,+0𝔽 is used.
The end argument is optional. If it is not provided, the length of thethisvalue is used.
Note 2
If start is negative, it is treated aslength + startwhere length is the length of the array. If end is negative, it is treated aslength + end.
When the fill method is called, the following steps are taken:
The fill function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returnstrue. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.
When the filter method is called, the following steps are taken:
The filter function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returnstrue. If such an element is found, find immediately returns that element value. Otherwise, find returnsundefined.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of predicate. If it is not provided,undefinedis used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by find is set before the first call to predicate. Elements that are appended to the array after the call to find begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them; elements that are deleted after the call to find begins and before being visited are not visited.
When the find method is called, the following steps are taken:
Let testResult be ! ToBoolean(?Call(predicate, thisArg, « kValue,𝔽(k), O »)).
If testResult istrue, return kValue.
Set k to k + 1.
Returnundefined.
Note 2
The find function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returnstrue. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of predicate. If it is not provided,undefinedis used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by findIndex is set before the first call to predicate. Elements that are appended to the array after the call to findIndex begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them; elements that are deleted after the call to findIndex begins and before being visited are not visited.
When the findIndex method is called, the following steps are taken:
The findIndex function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.10 Array.prototype.flat ( [ depth ] )
When the flat method is called, the following steps are taken:
The abstract operation FlattenIntoArray takes arguments target, source, sourceLen (a non-negativeinteger), start (a non-negativeinteger), and depth (a non-negativeintegeror +∞) and optional arguments mapperFunction and thisArg. It performs the following steps when called:
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.
When the forEach method is called, the following steps are taken:
Perform ? Call(callbackfn, thisArg, « kValue,𝔽(k), O »).
Set k to k + 1.
Returnundefined.
Note 2
The forEach function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
includes compares searchElement to the elements of the array, in ascending order, using theSameValueZeroalgorithm, and if found at any position, returnstrue; otherwise,falseis returned.
The optional second argument fromIndex defaults to+0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array,falseis returned, i.e. the array will not be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽, the whole array will be searched.
When the includes method is called, the following steps are taken:
The includes function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
The includes method intentionally differs from the similar indexOf method in two ways. First, it uses theSameValueZeroalgorithm, instead ofIsStrictlyEqual, allowing it to detectNaNarray elements. Second, it does not skip missing array elements, instead treating them asundefined.
indexOf compares searchElement to the elements of the array, in ascending order, using theIsStrictlyEqualalgorithm, and if found at one or more indices, returns the smallest such index; otherwise,-1𝔽 is returned.
Note 1
The optional second argument fromIndex defaults to+0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array,-1𝔽 is returned, i.e. the array will not be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽, the whole array will be searched.
When the indexOf method is called, the following steps are taken:
The indexOf function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.15 Array.prototype.join ( separator )
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
When the join method is called, the following steps are taken:
The join function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.16 Array.prototype.keys ( )
When the keys method is called, the following steps are taken:
lastIndexOf compares searchElement to the elements of the array in descending order using theIsStrictlyEqualalgorithm, and if found at one or more indices, returns the largest such index; otherwise,-1𝔽 is returned.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽,-1𝔽 is returned.
When the lastIndexOf method is called, the following steps are taken:
The lastIndexOf function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.
When the map method is called, the following steps are taken:
The map function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.19 Array.prototype.pop ( )
Note 1
The last element of the array is removed from the array and returned.
When the pop method is called, the following steps are taken:
The pop function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.20 Array.prototype.push ( ...items )
Note 1
The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.
When the push method is called, the following steps are taken:
The push function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was supplied, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is aTypeErrorif the array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.
When the reduce method is called, the following steps are taken:
Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
Set k to k + 1.
Return accumulator.
Note 2
The reduce function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was supplied, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is aTypeErrorif the array contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.
When the reduceRight method is called, the following steps are taken:
Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
Set k to k - 1.
Return accumulator.
Note 2
The reduceRight function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.23 Array.prototype.reverse ( )
Note 1
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.
When the reverse method is called, the following steps are taken:
Assert: lowerExists and upperExists are bothfalse.
No action is required.
Set lower to lower + 1.
Return O.
Note 2
The reverse function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.24 Array.prototype.shift ( )
The first element of the array is removed from the array and returned.
When the shift method is called, the following steps are taken:
The shift function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.25 Array.prototype.slice ( start, end )
The slice method returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end isundefined). If start is negative, it is treated aslength + startwhere length is the length of the array. If end is negative, it is treated aslength + endwhere length is the length of the array.
When the slice method is called, the following steps are taken:
The explicit setting of the"length"property of the result Array in step15was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting"length"became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 2
The slice function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returnstrue. If such an element is found, some immediately returnstrue. Otherwise, some returnsfalse. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returnsfalse.
When the some method is called, the following steps are taken:
Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
If testResult istrue, returntrue.
Set k to k + 1.
Returnfalse.
Note 2
The some function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.27 Array.prototype.sort ( comparefn )
The elements of this array are sorted. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparefn is notundefined, it should be a function that accepts two arguments x and y and returns a negative Number if x < y, a positive Number if x > y, or a zero otherwise.
When the sort method is called, the following steps are taken:
If comparefn is notundefinedandIsCallable(comparefn) isfalse, throw aTypeErrorexception.
The sort order is the ordering, after completion of this function, of theinteger-indexedproperty values of obj whoseintegerindexes are less than len. The result of the sort function is then determined as follows:
If comparefn is notundefinedand is not a consistent comparison function for the elements of items (see below).
If comparefn isundefinedandSortComparedoes not act as a consistent comparison function.
If comparefn isundefinedand all applications ofToString, to any specific value passed as an argument toSortCompare, do not produce the same result.
Unless the sort order is specified above to beimplementation-defined, items must satisfy all of the following conditions after executing step8of the algorithm above:
There must be some mathematical permutation π of the non-negative integers less than itemCount, such that for every non-negativeintegerj less than itemCount, the elementold[j]is exactly the same asnew[π(j)].
Then for all non-negative integers j and k, each less than itemCount, ifSortCompare(old[j], old[k]) < 0(seeSortComparebelow), thenπ(j) < π(k).
Here the notationold[j]is used to refer toitems[j]before step8is executed, and the notationnew[j]to refer toitems[j]after step8has been executed.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notationa <CFbmeanscomparefn(a, b) < 0;a =CFbmeanscomparefn(a, b) = 0(of either sign); anda >CFbmeanscomparefn(a, b) > 0.
Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore,Type(v) is Number, and v is notNaN. Note that this implies that exactly one of a <CFb, a =CFb, and a >CFb will be true for a given pair of a and b.
Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
a =CFa (reflexivity)
If a =CFb, then b =CFa (symmetry)
If a =CFb and b =CFc, then a =CFc (transitivity of =CF)
If a <CFb and b <CFc, then a <CFc (transitivity of <CF)
If a >CFb and b >CFc, then a >CFc (transitivity of >CF)
Note 1
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
Note 2
The sort function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.27.1 SortCompare ( x, y )
The abstract operation SortCompare takes arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. It performs the following steps when called:
If x and y are bothundefined, return+0𝔽.
If x isundefined, return1𝔽.
If y isundefined, return-1𝔽.
If comparefn is notundefined, then
Let v be ? ToNumber(?Call(comparefn,undefined, « x, y »)).
Because non-existent property values always compare greater thanundefinedproperty values, andundefinedalways compares greater than any other value,undefinedproperty values always sort to the end of the result, followed by non-existent property values.
Note 2
Method calls performed by theToStringabstract operationsin steps5and6have the potential to cause SortCompare to not behave as a consistent comparison function.
The deleteCount elements of the array starting atinteger indexstart are replaced by the elements of items. An Array object containing the deleted elements (if any) is returned.
When the splice method is called, the following steps are taken:
The explicit setting of the"length"property of the result Array in step20was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting"length"became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The splice function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
When the toLocaleString method is called, the following steps are taken:
Let separator be the String value for the list-separator String appropriate for thehost environment's current locale (this is derived in animplementation-definedway).
The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in animplementation-definedlocale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.
Note 3
The toLocaleString function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.30 Array.prototype.toString ( )
When the toString method is called, the following steps are taken:
The toString function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.31 Array.prototype.unshift ( ...items )
The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
When the unshift method is called, the following steps are taken:
The unshift function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.32 Array.prototype.values ( )
When the values method is called, the following steps are taken:
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
Note
The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.
23.1.4 Properties of Array Instances
Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from theArray prototype object.
Array instances have a"length"property, and a set of enumerable properties witharray indexnames.
23.1.4.1 length
The"length"property of an Array instance is adata propertywhose value is always numerically greater than the name of every configurable own property whose name is anarray index.
The"length"property initially has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
Note
Reducing the value of the"length"property has the side-effect of deleting own array elements whosearray indexis between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the"length"property of an Array object to a value that is numerically less than or equal to the largest numeric ownproperty nameof an existing non-configurablearray-indexedproperty of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric ownproperty name. See10.4.2.1.
23.1.5 Array Iterator Objects
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a namedconstructorfor Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.
23.1.5.1 CreateArrayIterator ( array, kind )
The abstract operation CreateArrayIterator takes arguments array and kind. It is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:
The initial value of the@@toStringTagproperty is the String value"Array Iterator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
23.2 TypedArray Objects
TypedArray objects present an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the underlying binary scalar data type that all elements of a TypedArray instance have. There is a distinct TypedArrayconstructor, listed inTable 63, for each of the supported element types. EachconstructorinTable 63has a corresponding distinct prototype object.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
23.2.2.4 get %TypedArray% [ @@species ]
%TypedArray%[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
23.2.3.1 get %TypedArray%.prototype.buffer
%TypedArray%.prototype.buffer is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
23.2.3.2 get %TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
%TypedArray%.prototype.byteOffset is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
The interpretation and use of the arguments of%TypedArray%.prototype.lastIndexOf are the same as for Array.prototype.lastIndexOf as defined in23.1.3.17.
When the lastIndexOf method is called, the following steps are taken:
This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.
23.2.3.18 get %TypedArray%.prototype.length
%TypedArray%.prototype.length is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
The interpretation and use of the arguments of%TypedArray%.prototype.reduceRight are the same as for Array.prototype.reduceRight as defined in23.1.3.22.
When the reduceRight method is called, the following steps are taken:
%TypedArray%.prototype.set is a function whose behaviour differs based upon the type of its first argument.
This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.
Sets multiple values in this TypedArray, reading the values from source. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
When the set method is called, the following steps are taken:
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray object), targetOffset (a non-negativeintegeror +∞), and source (a TypedArray object). It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:
Assert: source is an Object that has a [[TypedArrayName]] internal slot.
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray object), targetOffset (a non-negativeintegeror +∞), and source (an ECMAScript value other than a TypedArray object). It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:
Set targetByteIndex to targetByteIndex + targetElementSize.
23.2.3.24 %TypedArray%.prototype.slice ( start, end )
The interpretation and use of the arguments of%TypedArray%.prototype.slice are the same as for Array.prototype.slice as defined in23.1.3.25. The following steps are taken:
When the slice method is called, the following steps are taken:
%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in23.1.3.27. The implementation of the%TypedArray%.prototype.sort specification may be optimized with the knowledge that thethisvalue is an object that has a fixed length and whoseinteger-indexedproperties are not sparse.
This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of steps1–3in23.1.3.27:
If comparefn is notundefinedandIsCallable(comparefn) isfalse, throw aTypeErrorexception.
The following version ofSortCompareis used by%TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in23.1.3.27.
The abstract operation TypedArraySortCompare takes arguments x and y. It also has access to the comparefn and buffer values of the current invocation of the sort method. It performs the following steps when called:
Assert: BothType(x) andType(y) are Number or both are BigInt.
If comparefn is notundefined, then
Let v be ? ToNumber(?Call(comparefn,undefined, « x, y »)).
BecauseNaNalways compares greater than any other value,NaNproperty values always sort to the end of the result when comparefn is not provided.
23.2.3.27 %TypedArray%.prototype.subarray ( begin, end )
Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
When the subarray method is called, the following steps are taken:
%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in23.1.3.29except that thethisvalue's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of"length". The implementation of the algorithm may be optimized with the knowledge that thethisvalue is an object that has a fixed length and whoseinteger-indexedproperties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic.ValidateTypedArrayis applied to thethisvalue prior to evaluating the algorithm. If its result is anabrupt completionthat exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.
23.2.3.32 get %TypedArray%.prototype [ @@toStringTag ]
%TypedArray%.prototype[@@toStringTag] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:
The abstract operation TypedArraySpeciesCreate takes arguments exemplar and argumentList. It is used to specify the creation of a new TypedArray object using aconstructorfunction that is derived from exemplar. It performs the following steps when called:
Assert: exemplar is an Object that has [[TypedArrayName]] and [[ContentType]] internal slots.
Let defaultConstructor be the intrinsic object listed in column one ofTable 63for exemplar.[[TypedArrayName]].
The abstract operation TypedArrayCreate takes arguments constructor and argumentList. It is used to specify the creation of a new TypedArray object using aconstructorfunction. It performs the following steps when called:
Let newTypedArray be ? Construct(constructor, argumentList).
is an intrinsic object that has the structure described below, differing only in the name used as theconstructorname instead of TypedArray, inTable 63.
is a function whose behaviour differs based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArrayconstructorto create and initialize the subclass instance with the internal state necessary to support the%TypedArray%.prototype built-in methods.
has a"length"property whose value is3𝔽.
23.2.5.1TypedArray ( ...args )
Each TypedArrayconstructorperforms the following steps when called:
If NewTarget isundefined, throw aTypeErrorexception.
The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArrayconstructorinTable 63), newTarget, and defaultProto and optional argument length (a non-negativeinteger). It is used to validate and create an instance of a TypedArrayconstructor. If the length argument is passed, an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by TypedArray. It performs the following steps when called:
23.2.5.1.2 InitializeTypedArrayFromTypedArray ( O, srcArray )
The abstract operation InitializeTypedArrayFromTypedArray takes arguments O (a TypedArray object) and srcArray (a TypedArray object). It performs the following steps when called:
Assert: O is an Object that has a [[TypedArrayName]] internal slot.
Assert: srcArray is an Object that has a [[TypedArrayName]] internal slot.
Set srcByteIndex to srcByteIndex + srcElementSize.
Set targetByteIndex to targetByteIndex + elementSize.
Set count to count - 1.
Set O.[[ViewedArrayBuffer]] to data.
Set O.[[ByteLength]] to byteLength.
Set O.[[ByteOffset]] to 0.
Set O.[[ArrayLength]] to elementLength.
23.2.5.1.3 InitializeTypedArrayFromArrayBuffer ( O, buffer, byteOffset, length )
The abstract operation InitializeTypedArrayFromArrayBuffer takes arguments O (a TypedArray object), buffer (an ArrayBuffer object), byteOffset (anECMAScript language value), and length (anECMAScript language value). It performs the following steps when called:
Assert: O is an Object that has a [[TypedArrayName]] internal slot.
Assert: buffer is an Object that has an [[ArrayBufferData]] internal slot.
Let constructorName be the String value of O.[[TypedArrayName]].
Let elementSize be the Element Size value specified inTable 63for constructorName.
Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
If length isundefined, then
If bufferByteLengthmoduloelementSize ≠ 0, throw aRangeErrorexception.
Let newByteLength be bufferByteLength - offset.
If newByteLength < 0, throw aRangeErrorexception.
Else,
Let newByteLength be newLength × elementSize.
If offset + newByteLength > bufferByteLength, throw aRangeErrorexception.
Set O.[[ViewedArrayBuffer]] to buffer.
Set O.[[ByteLength]] to newByteLength.
Set O.[[ByteOffset]] to offset.
Set O.[[ArrayLength]] to newByteLength / elementSize.
23.2.5.1.4 InitializeTypedArrayFromList ( O, values )
The abstract operation InitializeTypedArrayFromList takes arguments O (a TypedArray object) and values (aListof ECMAScript language values). It performs the following steps when called:
Assert: O is an Object that has a [[TypedArrayName]] internal slot.
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray object) and arrayLike (an Object that is neither a TypedArray object nor an ArrayBuffer object). It performs the following steps when called:
Assert: O is an Object that has a [[TypedArrayName]] internal slot.
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray object) and length (a non-negativeinteger). It allocates and associates an ArrayBuffer with O. It performs the following steps when called:
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
23.2.7.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified inTable 63for TypedArray.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
23.2.7.2TypedArray.prototype.constructor
The initial value of a TypedArray.prototype.constructor is the corresponding%TypedArray%intrinsic object.
23.2.8 Properties of TypedArray Instances
TypedArray instances areInteger-Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 Keyed Collections
24.1 Map Objects
Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using theSameValueZerocomparison algorithm.
Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.
is the initial value of the"Map"property of theglobal object.
creates and initializes a new Map object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Mapconstructorto create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.
24.1.1.1 Map ( [ iterable ] )
When the Map function is called with optional argument iterable, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
If the parameter iterable is present, it is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
The abstract operation AddEntriesFromIterable takes arguments target, iterable, and adder (afunction object). adder will be invoked, with target as the receiver. It performs the following steps when called:
The parameter iterable is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
24.1.2.2 get Map [ @@species ]
Map[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
Methods that create derived collection objects should call@@speciesto determine theconstructorto use to create the derived objects. Subclassconstructormay over-ride@@speciesto change the defaultconstructorassignment.
For eachRecord{ [[Key]], [[Value]] } p of entries, do
If p.[[Key]] is notemptyandSameValueZero(p.[[Key]], key) istrue, then
Set p.[[Key]] toempty.
Set p.[[Value]] toempty.
Returntrue.
Returnfalse.
Note
The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
For eachRecord{ [[Key]], [[Value]] } e of entries, do
If e.[[Key]] is notempty, then
Perform ? Call(callbackfn, thisArg, « e.[[Value]], e.[[Key]], M »).
Returnundefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.
The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the"entries"property.
24.1.3.13 Map.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"Map".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.1.4 Properties of Map Instances
Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.
24.1.5 Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a namedconstructorfor Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.
24.1.5.1 CreateMapIterator ( map, kind )
The abstract operation CreateMapIterator takes arguments map and kind. It is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:
The initial value of the@@toStringTagproperty is the String value"Map Iterator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.2 Set Objects
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using theSameValueZerocomparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.
is the initial value of the"Set"property of theglobal object.
creates and initializes a new Set object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Setconstructorto create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.
24.2.1.1 Set ( [ iterable ] )
When the Set function is called with optional argument iterable, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
24.2.2.2 get Set [ @@species ]
Set[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
Methods that create derived collection objects should call@@speciesto determine theconstructorto use to create the derived objects. Subclassconstructormay over-ride@@speciesto change the defaultconstructorassignment.
If e is notemptyandSameValueZero(e, value) istrue, then
Replace the element of entries whose value is e with an element whose value isempty.
Returntrue.
Returnfalse.
Note
The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.
callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.
The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.
The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the"values"property.
24.2.3.12 Set.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"Set".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.2.4 Properties of Set Instances
Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.
24.2.5 Set Iterator Objects
A Set Iterator is anordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a namedconstructorfor Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.
24.2.5.1 CreateSetIterator ( set, kind )
The abstract operation CreateSetIterator takes arguments set and kind. It is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:
The initial value of the@@toStringTagproperty is the String value"Set Iterator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.3 WeakMap Objects
WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. In certain conditions, objects which are notliveare removed as WeakMap keys, as described in9.10.3.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
is the initial value of the"WeakMap"property of theglobal object.
creates and initializes a new WeakMap object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMapconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
When the WeakMap function is called with optional argument iterable, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
If the parameter iterable is present, it is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.
For eachRecord{ [[Key]], [[Value]] } p of entries, do
If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, then
Set p.[[Key]] toempty.
Set p.[[Value]] toempty.
Returntrue.
Returnfalse.
Note
The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
IfType(key) is not Object, throw aTypeErrorexception.
For eachRecord{ [[Key]], [[Value]] } p of entries, do
If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, then
Set p.[[Value]] to value.
Return M.
Let p be theRecord{ [[Key]]: key, [[Value]]: value }.
Append p as the last element of entries.
Return M.
24.3.3.6 WeakMap.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"WeakMap".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.3.4 Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
24.4 WeakSet Objects
WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. In certain conditions, objects which are notliveare removed as WeakSet elements, as described in9.10.3.
An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.
WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.
is the initial value of the"WeakSet"property of theglobal object.
creates and initializes a new WeakSet object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSetconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
When the WeakSet function is called with optional argument iterable, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
If e is notemptyandSameValue(e, value) istrue, then
Replace the element of entries whose value is e with an element whose value isempty.
Returntrue.
Returnfalse.
Note
The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
If e is notemptyandSameValue(e, value) istrue, returntrue.
Returnfalse.
24.4.3.5 WeakSet.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"WeakSet".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
24.4.4 Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
25 Structured Data
25.1 ArrayBuffer Objects
25.1.1 Notation
The descriptions below in this section,25.4, and29use the read-modify-write modification function internal data structure.
A read-modify-write modification function is a mathematical function that is notationally represented as an abstract closure that takes two Lists of byte values as arguments and returns aListof byte values. These abstract closures satisfy all of the following properties:
They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure, mathematical function, the following editorial conventions are recommended:
They do not access, directly or transitively via invokedabstract operationsand abstract closures, any language or specification values except their parameters and captured values.
The abstract operation AllocateArrayBuffer takes arguments constructor and byteLength (a non-negativeinteger). It is used to create an ArrayBuffer object. It performs the following steps when called:
Let obj be ? OrdinaryCreateFromConstructor(constructor,"%ArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).
Detaching an ArrayBuffer instance disassociates theData Blockused as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScripthostor implementation may define such operations.
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer object), srcByteOffset (a non-negativeinteger), srcLength (a non-negativeinteger), and cloneConstructor (aconstructor). It creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at srcByteOffset and continuing for srcLength bytes. It performs the following steps when called:
Assert:Type(srcBuffer) is Object and srcBuffer has an [[ArrayBufferData]] internal slot.
The abstract operation IsUnsignedElementType takes argument type. It verifies if the argument type is an unsignedTypedArray element type. It performs the following steps when called:
If type isUint8,Uint8C,Uint16,Uint32, orBigUint64, returntrue.
Returnfalse.
25.1.2.6 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type. It verifies if the argument type is anIntegerTypedArray element typenot includingUint8C. It performs the following steps when called:
If type isInt8,Uint8,Int16,Uint16,Int32, orUint32, returntrue.
Returnfalse.
25.1.2.7 IsBigIntElementType ( type )
The abstract operation IsBigIntElementType takes argument type. It verifies if the argument type is a BigIntTypedArray element type. It performs the following steps when called:
If type isBigUint64orBigInt64, returntrue.
Returnfalse.
25.1.2.8 IsNoTearConfiguration ( type, order )
The abstract operation IsNoTearConfiguration takes arguments type and order. It performs the following steps when called:
The abstract operation RawBytesToNumeric takes arguments type (aTypedArray element type), rawBytes (aList), and isLittleEndian (a Boolean). It performs the following steps when called:
Let elementSize be the Element Size value specified inTable 63for Element Type type.
If isLittleEndian isfalse, reverse the order of the elements of rawBytes.
If type isFloat32, then
Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of anIEEE 754-2019binary32 value.
Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
If ! IsBigIntElementType(type) istrue, return the BigInt value that corresponds to intValue.
Otherwise, return theNumber valuethat corresponds to intValue.
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negativeinteger), type (aTypedArray element type), isTypedArray (a Boolean), and order (eitherSeqCstorUnordered) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
If isTypedArray istrueandIsNoTearConfiguration(type, order) istrue, let noTear betrue; otherwise let noTear befalse.
Let rawValue be aListof length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
The abstract operation NumericToRawBytes takes arguments type (aTypedArray element type), value (a BigInt or a Number), and isLittleEndian (a Boolean). It performs the following steps when called:
If type isFloat32, then
Let rawBytes be aListwhose elements are the 4 bytes that are the result of converting value toIEEE 754-2019binary32 format using roundTiesToEven mode. If isLittleEndian isfalse, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN, rawBytes may be set to any implementation chosenIEEE 754-2019binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaNvalue.
Else if type isFloat64, then
Let rawBytes be aListwhose elements are the 8 bytes that are theIEEE 754-2019binary64 format encoding of value. If isLittleEndian isfalse, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN, rawBytes may be set to any implementation chosenIEEE 754-2019binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaNvalue.
Else,
Let n be the Element Size value specified inTable 63for Element Type type.
Let convOp be the abstract operation named in the Conversion Operation column inTable 63for Element Type type.
Let rawBytes be aListwhose elements are the n-byte binary encoding of intValue. If isLittleEndian isfalse, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Else,
Let rawBytes be aListwhose elements are the n-byte binary two's complement encoding of intValue. If isLittleEndian isfalse, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negativeinteger), type (aTypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (one ofSeqCst,Unordered, orInit) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer object or a SharedArrayBuffer object), byteIndex (a non-negativeinteger), type (aTypedArray element type), value (a Number or a BigInt), and op (aread-modify-write modification function) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
Let rawBytesRead be aListof length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
Let rmwEvent beReadModifyWriteSharedMemory{ [[Order]]:SeqCst, [[NoTear]]:true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
Append rmwEvent to eventList.
AppendChosen Value Record{ [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
Else,
Let rawBytesRead be aListof length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex].
Let rawBytesModified be op(rawBytesRead, rawBytes).
Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
is the initial value of the"ArrayBuffer"property of theglobal object.
creates and initializes a new ArrayBuffer object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBufferconstructorto create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.
25.1.3.1 ArrayBuffer ( length )
When the ArrayBuffer function is called with argument length, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
25.1.4.3 get ArrayBuffer [ @@species ]
ArrayBuffer[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.
25.1.5 Properties of the ArrayBuffer Prototype Object
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.5.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
The initial value of the@@toStringTagproperty is the String value"ArrayBuffer".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
25.1.6 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from theArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] isnullare considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other thanundefinedneed to have allDetachArrayBuffercalls passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding environments, not by algorithms in this specification.
25.2 SharedArrayBuffer Objects
25.2.1 Abstract Operations for SharedArrayBuffer Objects
The abstract operation AllocateSharedArrayBuffer takes arguments constructor and byteLength (a non-negativeinteger). It is used to create a SharedArrayBuffer object. It performs the following steps when called:
Let obj be ? OrdinaryCreateFromConstructor(constructor,"%SharedArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
The abstract operation IsSharedArrayBuffer takes argument obj. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
Assert:Type(obj) is Object and obj has an [[ArrayBufferData]] internal slot.
is the initial value of the"SharedArrayBuffer"property of theglobal object, if that property is present (see below).
creates and initializes a new SharedArrayBuffer object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBufferconstructorto create and initialize subclass instances with the internal state necessary to support the SharedArrayBuffer.prototype built-in methods.
Whenever ahostdoes not provide concurrent access to SharedArrayBuffer objects it may omit the"SharedArrayBuffer"property of theglobal object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is nevernull.
25.2.2.1 SharedArrayBuffer ( length )
When the SharedArrayBuffer function is called with argument length, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.4.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
The initial value of the@@toStringTagproperty is the String value"SharedArrayBuffer".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
25.2.5 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from theSharedArrayBuffer prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 GetViewValue ( view, requestIndex, isLittleEndian, type )
The abstract operation GetViewValue takes arguments view, requestIndex, isLittleEndian, and type. It is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:
25.3.1.2 SetViewValue ( view, requestIndex, isLittleEndian, type, value )
The abstract operation SetViewValue takes arguments view, requestIndex, isLittleEndian, type, and value. It is used by functions on DataView instances to store values into the view's buffer. It performs the following steps when called:
is the initial value of the"DataView"property of theglobal object.
creates and initializes a new DataView object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataViewconstructorto create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.
The initial value of the@@toStringTagproperty is the String value"DataView".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from theDataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataViewconstructor.
25.4 The Atomics Object
The Atomics object:
is %Atomics%.
is the initial value of the"Atomics"property of theglobal object.
does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agentprograms that communicate through shared memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory communication are provided by thememory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of thememory modelsection.
25.4.1 WaiterList Objects
A WaiterList is a semantic object that contains an ordered list of those agents that are waiting on a location (block, i) in shared memory; block is aShared Data Blockand i a byte offset into the memory of block. A WaiterList object also optionally contains aSynchronize eventdenoting the previous leaving of its critical section.
Initially a WaiterList object has an empty list and noSynchronize event.
Theagent clusterhas a store of WaiterList objects; the store is indexed by (block, i). WaiterLists areagent-independent: a lookup in the store of WaiterLists by (block, i) will result in the same WaiterList object in anyagentin theagent cluster.
Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a singleagentmay enter a WaiterList's critical section at one time. Entering and leaving a WaiterList's critical section is controlled by theabstract operationsEnterCriticalSectionandLeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving theSynchronize event—may only be performed by agents that have entered the WaiterList's critical section.
The abstract operation ValidateIntegerTypedArray takes argument typedArray and optional argument waitable (a Boolean). It performs the following steps when called:
If accessIndex ≥ length, throw aRangeErrorexception.
Let arrayTypeName be typedArray.[[TypedArrayName]].
Let elementSize be the Element Size value specified inTable 63for arrayTypeName.
Let offset be typedArray.[[ByteOffset]].
Return (accessIndex × elementSize) + offset.
25.4.2.3 GetWaiterList ( block, i )
The abstract operation GetWaiterList takes arguments block (aShared Data Block) and i (a non-negativeinteger). It performs the following steps when called:
Append (leaveEvent, enterEvent) to eventsRecord.[[AgentSynchronizesWith]].
EnterCriticalSection has contention when anagentattempting to enter thecritical sectionmust wait for anotheragentto leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause anagentto wait indefinitely.
25.4.2.5 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (aWaiterList). It performs the following steps when called:
The abstract operation SuspendAgent takes arguments WL (aWaiterList), W (anagentsignifier), and timeout (a non-negativeinteger). It performs the following steps when called:
PerformLeaveCriticalSection(WL) and suspend W for up to timeout milliseconds, performing the combined operation in such a way that a notification that arrives after thecritical sectionis exited but before the suspension takes effect is not lost. W can notify either because the timeout expired or because it was notified explicitly by anotheragentcallingNotifyWaiter(WL, W), and not for any other reasons at all.
The embedding may delay notifying W, e.g. for resource management reasons, but W must eventually be notified in order to guarantee forward progress.
25.4.2.11 AtomicReadModifyWrite ( typedArray, index, value, op )
The abstract operation AtomicReadModifyWrite takes arguments typedArray, index, value, and op (aread-modify-write modification function). op takes twoListof byte values arguments and returns aListof byte values. This operation atomically loads a value, combines it with another value, and stores the result of the combination. It returns the loaded value. It performs the following steps when called:
NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
Let elementType be the Element Type value inTable 63for arrayTypeName.
The abstract operation ByteListBitwiseOp takes arguments op (a sequence of Unicode code points), xBytes (aListof byte values), and yBytes (aListof byte values). The operation atomically performs a bitwise operation on all byte values of the arguments and returns aListof byte values. It performs the following steps when called:
If op is &, let resultByte be the result of applying the bitwise AND operation to xByte and yByte.
Else if op is ^, let resultByte be the result of applying the bitwise exclusive OR (XOR) operation to xByte and yByte.
Else, op is |. Let resultByte be the result of applying the bitwise inclusive OR operation to xByte and yByte.
Set i to i + 1.
Append resultByte to the end of result.
Return result.
25.4.2.13 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (aListof byte values) and yBytes (aListof byte values). It performs the following steps when called:
If xBytes and yBytes do not have the same number of elements, returnfalse.
Let i be 0.
For each element xByte of xBytes, do
Let yByte be yBytes[i].
If xByte ≠ yByte, returnfalse.
Set i to i + 1.
Returntrue.
25.4.3 Atomics.add ( typedArray, index, value )
The following steps are taken:
Let type be the Element Type value inTable 63for typedArray.[[TypedArrayName]].
Let add be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
Let and be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
Let elementType be the Element Type value inTable 63for arrayTypeName.
Let elementSize be the Element Size value specified inTable 63for Element Type elementType.
Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
Let rawBytesRead be aListof length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
NOTE: The comparison of the expected value and the read value is performed outside of theread-modify-write modification functionto avoid needlessly strong synchronization when the expected value is not equal to the read value.
IfByteListEqual(rawBytesRead, expectedBytes) istrue, then
Let second be a newread-modify-write modification functionwith parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
Return newBytes.
Let event beReadModifyWriteSharedMemory{ [[Order]]:SeqCst, [[NoTear]]:true, [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
25.4.6 Atomics.exchange ( typedArray, index, value )
The following steps are taken:
Let second be a newread-modify-write modification functionwith parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
Atomics.isLockFree() is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the callingagentacquiring a lock outside the n bytes comprising the datum, then Atomics.isLockFree(n) will returntrue. High-performance algorithms will use Atomics.isLockFree to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.
Atomics.isLockFree(4) always returnstrueas that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.
Regardless of the value of Atomics.isLockFree, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").
NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toValidateAtomicAccesson the preceding line can have arbitrary side effects, which could cause the buffer to become detached.
Let arrayTypeName be typedArray.[[TypedArrayName]].
Let elementType be the Element Type value inTable 63for arrayTypeName.
Let or be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
Let elementType be the Element Type value inTable 63for arrayTypeName.
Let subtract be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
Let xor be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, andnull.
The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returnsundefinedthen the property is deleted from the result.
Parse ! StringToCodePoints(jsonString) as a JSON text as specified in ECMA-404. Throw aSyntaxErrorexception if it is not a valid JSON text as defined in that specification.
Valid JSON text is a subset of the ECMAScriptPrimaryExpressionsyntax. Step2verifies that jsonString conforms to that subset, and step10asserts that that parsing and evaluation returns a value of an appropriate type.
However, because13.2.5.5behaves differently during JSON.parse, the same source text can produce different results when evaluated as aPrimaryExpressionrather than as JSON. Furthermore, the Early Error for duplicate"__proto__"properties in object literals, which likewise does not apply during JSON.parse, means that not all texts accepted by JSON.parse are valid as aPrimaryExpression, despite matching the grammar.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.
Note 2
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )
The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, orundefined. It can take three parameters. The value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number ornull. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.
The"length"property of the stringify function is3𝔽.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw aTypeErrorexception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
Thenullvalue is rendered in JSON text as the String"null".
Theundefinedvalue is not rendered.
Thetruevalue is rendered in JSON text as the String"true".
Thefalsevalue is rendered in JSON text as the String"false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by callingToString(number).NaNandInfinityregardless of sign are represented as the String"null".
Note 5
Values that do not have a JSON representation (such asundefinedand functions) do not produce a String. Instead they produce theundefinedvalue. In arrays these values are represented as the String"null". In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key orproperty name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
The abstract operation QuoteJSONString takes argument value. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in6.1.4. It performs the following steps when called:
Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
Set product to thestring-concatenationof product and the code unit 0x0022 (QUOTATION MARK).
Return product.
Table 64: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
25.5.2.3 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit). It represents C as a Unicode escape sequence. It performs the following steps when called:
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let separator be thestring-concatenationof the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be thestring-concatenationof"{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"}".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepback.
Return final.
25.5.2.5 SerializeJSONArray ( state, value )
The abstract operation SerializeJSONArray takes arguments state and value. It serializes an array. It performs the following steps when called:
If state.[[Stack]] contains value, throw aTypeErrorexception because the structure is cyclical.
Append value to state.[[Stack]].
Let stepback be state.[[Indent]].
Set state.[[Indent]] to thestring-concatenationof state.[[Indent]] and state.[[Gap]].
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let separator be thestring-concatenationof the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be thestring-concatenationof"[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"]".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepback.
Return final.
Note
The representation of arrays includes only the elements between zero andarray.length - 1inclusive. Properties whose keys are notarray indexesare excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.
25.5.3 JSON [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"JSON".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
26 Managing Memory
26.1 WeakRef Objects
AWeakRefis an object that is used to refer to a target object without preserving it from garbage collection. WeakRefs can be dereferenced to allow access to the target object, if the target object hasn't been reclaimed by garbage collection.
is the initial value of the"WeakRef"property of theglobal object.
creates and initializes a new WeakRef object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakRef behaviour must include a super call to the WeakRefconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakRef.prototype built-in methods.
26.1.1.1 WeakRef ( target )
When the WeakRef function is called with argument target, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
IfType(target) is not Object, throw aTypeErrorexception.
If theWeakRefreturns a target Object that is notundefined, then this target object should not be garbage collected until the current execution of ECMAScript code has completed. TheAddToKeptObjectsoperation makes sure read consistency is maintained.
target = { foo: function() {} };
let weakRef = newWeakRef(target);
... later ...
if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate toundefinedthen the second deref cannot either.
26.1.3.3 WeakRef.prototype [ @@toStringTag ]
The initial value of the@@toStringTagproperty is the String value"WeakRef".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (aWeakRef). It performs the following steps when called:
This abstract operation is defined separately from WeakRef.prototype.deref strictly to make it possible to succinctly define liveness.
26.1.5 Properties of WeakRef Instances
WeakRefinstances are ordinary objects that inherit properties from theWeakRef prototype.WeakRefinstances also have a [[WeakRefTarget]] internal slot.
26.2 FinalizationRegistry Objects
AFinalizationRegistryis an object that manages registration and unregistration of cleanup operations that are performed when target objects are garbage collected.
is the initial value of the"FinalizationRegistry"property of theglobal object.
creates and initializes a new FinalizationRegistry object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified FinalizationRegistry behaviour must include a super call to the FinalizationRegistryconstructorto create and initialize the subclass instance with the internal state necessary to support the FinalizationRegistry.prototype built-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
When the FinalizationRegistry function is called with argument cleanupCallback, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
If unregisterToken is notundefined, throw aTypeErrorexception.
Set unregisterToken toempty.
Let cell be theRecord{ [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
Append cell to finalizationRegistry.[[Cells]].
Returnundefined.
Note
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] islivewhen cell is in finalizationRegistry.[[Cells]]; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] arelive. For example, registering an object with itself as its unregister token would not keep the object alive forever.
An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The Iterable interface includes the property described inTable 65:
Table 65: Iterable Interface Required Properties
Property
Value
Requirements
@@iterator
A function that returns an Iterator object.
The returned object must conform to the Iterator interface.
27.1.1.2 The Iterator Interface
An object that implements the Iterator interface must include the property inTable 66. Such objects may also implement the properties inTable 67.
Table 66: Iterator Interface Required Properties
Property
Value
Requirements
"next"
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose"done"property istrue, then all subsequent calls to the next method of that object should also return an IteratorResult object whose"done"property istrue. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 67: Iterator Interface Optional Properties
Property
Value
Requirements
"return"
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a"done"property whose value istrue, and a"value"property with the value passed as the argument of the return method. However, this requirement is not enforced.
"throw"
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a"done"property whose value istrue.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.
27.1.1.3 The AsyncIterable Interface
The AsyncIterable interface includes the properties described inTable 68:
The returned object must conform to the AsyncIterator interface.
27.1.1.4 The AsyncIterator Interface
An object that implements the AsyncIterator interface must include the properties inTable 69. Such objects may also implement the properties inTable 70.
A function that returns a promise for an IteratorResult object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. If a previous call to the next method of an AsyncIterator has returned a promise for an IteratorResult object whose"done"property istrue, then all subsequent calls to the next method of that object should also return a promise for an IteratorResult object whose"done"property istrue. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have a"value"property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
A function that returns a promise for an IteratorResult object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more next method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which will typically have a"done"property whose value istrue, and a"value"property with the value passed as the argument of the return method. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have a"value"property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the"value"property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced.
"throw"
A function that returns a promise for an IteratorResult object.
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument.
If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a"done"property whose value istrue. Additionally, it should have a"value"property whose value is not a promise (or "thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult interface includes the properties listed inTable 71:
Table 71: IteratorResult Interface Properties
Property
Value
Requirements
"done"
Eithertrueorfalse.
This is the result status of an iteratornext method call. If the end of the iterator was reached"done"istrue. If the end was not reached"done"isfalseand a value is available. If a"done"property (either own or inherited) does not exist, it is considered to have the valuefalse.
If done isfalse, this is the current iteration element value. If done istrue, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value,"value"isundefined. In that case, the"value"property may be absent from the conforming object if it does not inherit an explicit"value"property.
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.
The value of the"name"property of this function is"[Symbol.asyncIterator]".
27.1.4 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a namedconstructorfor Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by theCreateAsyncFromSyncIteratorabstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord. It is used to create an async iteratorRecordfrom a synchronous iteratorRecord. It performs the following steps when called:
27.1.4.3 Properties of Async-from-Sync Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that inherit properties from the%AsyncFromSyncIteratorPrototype%intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed inTable 72. Async-from-Sync Iterator instances are not directly observable from ECMAScript code.
Table 72: Internal Slots of Async-from-Sync Iterator Instances
Internal Slot
Description
[[SyncIteratorRecord]]
ARecord, of the type returned byGetIterator, representing the original synchronous iterator which is being adapted.
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result and promiseCapability (aPromiseCapability Record). It performs the following steps when called:
NOTE: onFulfilled is used when processing the"value"property of an IteratorResult object in order to wait for its value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object.
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue aJobto call the function f.
A promise p is rejected if p.then(f, r) will immediately enqueue aJobto call the function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is aRecordvalue used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability Records are produced by theNewPromiseCapabilityabstract operation.
PromiseCapability Records have the fields listed inTable 73.
The PromiseReaction is aRecordvalue used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by thePerformPromiseThenabstract operation, and are used by theAbstract Closurereturned byNewPromiseReactionJob.
PromiseReaction records have the fields listed inTable 74.
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] isempty, a function that depends on the value of [[Type]] will be used instead.
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise. It performs the following steps when called:
Let alreadyResolved be theRecord{ [[Value]]:false}.
The abstract operation NewPromiseCapability takes argument C. It attempts to use C as aconstructorin the fashion of the built-in Promiseconstructorto create a Promise object and extract its resolve and reject functions. The Promise object plus the resolve and reject functions are used to initialize a newPromiseCapability Record. It performs the following steps when called:
NOTE: C is assumed to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor(see27.2.3.1).
Let promiseCapability be thePromiseCapability Record{ [[Promise]]:undefined, [[Resolve]]:undefined, [[Reject]]:undefined}.
Let executorClosure be a newAbstract Closurewith parameters (resolve, reject) that captures promiseCapability and performs the following steps when called:
If promiseCapability.[[Resolve]] is notundefined, throw aTypeErrorexception.
If promiseCapability.[[Reject]] is notundefined, throw aTypeErrorexception.
This abstract operation supports Promise subclassing, as it is generic on anyconstructorthat calls a passed executor function argument in the same way as the Promiseconstructor. It is used to generalize static methods of the Promiseconstructorto any subclass.
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x. It checks for the promise brand on an object. It performs the following steps when called:
The abstract operation TriggerPromiseReactions takes arguments reactions (aListof PromiseReaction Records) and argument. It enqueues a newJobfor each record in reactions. Each suchJobprocesses the [[Type]] and [[Handler]] of the PromiseReactionRecord, and if the [[Handler]] is notempty, calls it passing the given argument. If the [[Handler]] isempty, the behaviour is determined by the [[Type]]. It performs the following steps when called:
Thehost-definedabstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and operation ("reject"or"handle"). It allowshostenvironments to track promise rejections.
An implementation of HostPromiseRejectionTracker must conform to the following requirements:
The default implementation of HostPromiseRejectionTracker is to returnNormalCompletion(empty).
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its operation argument set to"reject".
When a handler is added to a rejected promise for the first time, it is called with its operation argument set to"handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
Note 2
If operation is"handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is"reject", since it is expected that rejections will be rare and not on hot code paths.
The abstract operation NewPromiseReactionJob takes arguments reaction and argument. It returns a newJobAbstract Closurethat applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler. It performs the following steps when called:
Let job be a newJobAbstract Closurewith no parameters that captures reaction and argument and performs the following steps when called:
NOTE: handlerRealm is nevernullunless the handler isundefined. When the handler is a revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error objects.
ThisJobuses the supplied thenable and its then method to resolve the given promise. This process must take place as aJobto ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.
is the initial value of the"Promise"property of theglobal object.
creates and initializes a new Promise object when called as aconstructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promiseconstructorto create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.
27.2.3.1 Promise ( executor )
When the Promise function is called with argument executor, the following steps are taken:
If NewTarget isundefined, throw aTypeErrorexception.
The executor argument must be afunction object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promiseconstructorhave the capability to actually resolve and reject the associated promise. Subclasses may have differentconstructorbehaviour that passes in customized values for resolve and reject.
The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.
The abstract operation PerformPromiseAll takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the following steps are taken:
The"length"property of a Promise.all resolve element function is1𝔽.
27.2.4.2 Promise.allSettled ( iterable )
The allSettled function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:
27.2.4.2.2Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:
The"length"property of a Promise.allSettled resolve element function is1𝔽.
27.2.4.2.3Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x, the following steps are taken:
The"length"property of a Promise.allSettled reject element function is1𝔽.
27.2.4.3 Promise.any ( iterable )
The any function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
The abstract operation PerformPromiseAny takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:
A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the following steps are taken:
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
27.2.4.5 Promise.race ( iterable )
The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.
Note 2
The race function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor. It also expects that itsthisvalue provides a resolve method.
The abstract operation PerformPromiseRace takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:
Perform ? Call(promiseCapability.[[Reject]],undefined, « r »).
Return promiseCapability.[[Promise]].
Note
The reject function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.
27.2.4.7 Promise.resolve ( x )
The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by thisconstructor.
Let C be thethisvalue.
IfType(C) is not Object, throw aTypeErrorexception.
The resolve function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.
27.2.4.7.1 PromiseResolve ( C, x )
The abstract operation PromiseResolve takes arguments C (aconstructor) and x (anECMAScript language value). It returns a new promise resolved with x. It performs the following steps when called:
Perform ? Call(promiseCapability.[[Resolve]],undefined, « x »).
Return promiseCapability.[[Promise]].
27.2.4.8 get Promise [ @@species ]
Promise[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:
Return thethisvalue.
The value of the"name"property of this function is"get [Symbol.species]".
Note
Promise prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.
The abstract operation PerformPromiseThen takes arguments promise, onFulfilled, and onRejected and optional argument resultCapability (aPromiseCapability Record). It performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. If resultCapability is passed, the result is stored by updating resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter. It performs the following steps when called:
The initial value of the@@toStringTagproperty is the String value"Promise".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.2.6 Properties of Promise Instances
Promise instances are ordinary objects that inherit properties from thePromise prototype object(the intrinsic,%Promise.prototype%). Promise instances are initially created with the internal slots described inTable 75.
Table 75: Internal Slots of Promise Instances
Internal Slot
Description
[[PromiseState]]
One ofpending,fulfilled, orrejected. Governs how a promise will react to incoming calls to its then method.
[[PromiseResult]]
The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is notpending.
[[PromiseFulfillReactions]]
AListof PromiseReaction records to be processed when/if the promise transitions from thependingstate to thefulfilledstate.
[[PromiseRejectReactions]]
AListof PromiseReaction records to be processed when/if the promise transitions from thependingstate to therejectedstate.
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.
creates and initializes a new GeneratorFunction object when called as a function rather than as aconstructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunctionconstructorto create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.
27.3.1.1 GeneratorFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.
When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
The initial value of the@@toStringTagproperty is the String value"GeneratorFunction".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the"length"property of Function instances given in20.2.4.1also applies to GeneratorFunction instances.
27.3.4.2 name
The specification for the"name"property of Function instances given in20.2.4.2also applies to GeneratorFunction instances.
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created anotherordinary objectis also created and is the initial value of the generator function's"prototype"property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator object when the generatorfunction objectis invoked using [[Call]].
This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
Note
Unlike Function instances, the object that is the value of the a GeneratorFunction's"prototype"property does not have a"constructor"property whose value is the GeneratorFunction instance.
creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as aconstructor. Thus the function call AsyncGeneratorFunction (...) is equivalent to the object creation expression new AsyncGeneratorFunction (...) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a super call to the AsyncGeneratorFunctionconstructorto create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.
27.4.1.1 AsyncGeneratorFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of an async generator function; any preceding arguments specify formal parameters.
When the AsyncGeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no "p" arguments, and where body might also not be provided), the following steps are taken:
The initial value of the@@toStringTagproperty is the String value"AsyncGeneratorFunction".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the"length"property is anintegral Numberthat indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its"length"property depends on the function.
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.4.4.2 name
The specification for the"name"property of Function instances given in20.2.4.2also applies to AsyncGeneratorFunction instances.
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created anotherordinary objectis also created and is the initial value of the async generator function's"prototype"property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator object when the generatorfunction objectis invoked using [[Call]].
This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
Note
Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's"prototype"property does not have a"constructor"property whose value is the AsyncGeneratorFunction instance.
27.5 Generator Objects
A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that is the initial value of the"prototype"property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic,%GeneratorFunction.prototype.prototype%.
27.5.1 Properties of the Generator Prototype Object
The abstract operation GeneratorStart takes arguments generator and generatorBody (aParse Nodeor anAbstract Closurewith no parameters). It performs the following steps when called:
Assert: The value of generator.[[GeneratorState]] isundefined.
Once a generator enters thecompletedstate it never leaves it and its associatedexecution contextis never resumed. Any execution state associated with generator can be discarded at this point.
If result.[[Type]] isnormal, let resultValue beundefined.
Else if result.[[Type]] isreturn, let resultValue be result.[[Value]].
Resume the suspended evaluation of genContext usingNormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
The abstract operation GeneratorResumeAbrupt takes arguments generator, abruptCompletion (aCompletion Recordwhose [[Type]] isreturnorthrow), and generatorBrand. It performs the following steps when called:
Once a generator enters thecompletedstate it never leaves it and its associatedexecution contextis never resumed. Any execution state associated with generator can be discarded at this point.
Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Set the code evaluation state of genContext such that when evaluation is resumed with aCompletionresumptionValue the following steps will be performed:
Return resumptionValue.
NOTE: This returns to the evaluation of theYieldExpressionthat originally called this abstract operation.
The abstract operation CreateIteratorFromClosure takes arguments closure (anAbstract Closurewith no parameters), generatorBrand, and generatorPrototype (an Object). It performs the following steps when called:
NOTE: closure can contain uses of theYieldshorthand to yield an IteratorResult object.
Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
An AsyncGenerator object is an instance of an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the object that is the initial value of the"prototype"property of the AsyncGenerator function that created the instance. AsyncGenerator instances indirectly inherit properties from the AsyncGenerator Prototype intrinsic,%AsyncGeneratorFunction.prototype.prototype%.
27.6.1 Properties of the AsyncGenerator Prototype Object
The initial value of the@@toStringTagproperty is the String value"AsyncGenerator".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.6.2 Properties of AsyncGenerator Instances
AsyncGenerator instances are initially created with the internal slots described below:
Table 77: Internal Slots of AsyncGenerator Instances
Internal Slot
Description
[[AsyncGeneratorState]]
The current execution state of the async generator. The possible values are:undefined,suspendedStart,suspendedYield,executing,awaiting-return, andcompleted.
[[AsyncGeneratorContext]]
Theexecution contextthat is used when executing the code of this async generator.
[[AsyncGeneratorQueue]]
AListofAsyncGeneratorRequestrecords which represent requests to resume the async generator. Except during state transitions, it is nonempty if and only if [[AsyncGeneratorState]] is eitherexecutingorawaiting-return.
[[GeneratorBrand]]
A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is alwaysempty.
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
An AsyncGeneratorRequest is aRecordvalue used to store information about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the corresponding promise.
The abstract operation AsyncGeneratorStart takes arguments generator and generatorBody (aParse Nodeor anAbstract Closurewith no parameters). It performs the following steps when called:
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator), completion (aCompletion Record), and promiseCapability (aPromiseCapability Record). It performs the following steps when called:
Let request beAsyncGeneratorRequest{ [[Completion]]: completion, [[Capability]]: promiseCapability }.
Append request to the end of generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an AsyncGenerator), completion (aCompletion Record), and done (a Boolean) and optional argument realm (aRealm Record). It performs the following steps when called:
The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and completion (aCompletion Record). It performs the following steps when called:
Assert: generator.[[AsyncGeneratorState]] is eithersuspendedStartorsuspendedYield.
Let genContext be generator.[[AsyncGeneratorContext]].
Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
The abstract operation AsyncGeneratorUnwrapYieldResumption takes argument resumptionValue (aCompletion Record). It performs the following steps when called:
If resumptionValue.[[Type]] is notreturn, returnCompletion(resumptionValue).
Set the code evaluation state of genContext such that when evaluation is resumed with aCompletionresumptionValue the following steps will be performed:
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator). It drains the generator's AsyncGeneratorQueue until it encounters anAsyncGeneratorRequestwhich holds a completion whose type isreturn. It performs the following steps when called:
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (anAbstract Closurewith no parameters), generatorBrand, and generatorPrototype (an Object). It performs the following steps when called:
NOTE: closure can contain uses of theAwaitshorthand and uses of theYieldshorthand to yield an IteratorResult object.
Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
creates and initializes a new AsyncFunction object when called as a function rather than as aconstructor. Thus the function call AsyncFunction(…) is equivalent to the object creation expression new AsyncFunction(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunctionconstructorto create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.
27.7.1.1 AsyncFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.
When the AsyncFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no p arguments, and where body might also not be provided), the following steps are taken:
The initial value of the@@toStringTagproperty is the String value"AsyncFunction".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they are not constructible.
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the"length"property of Function instances given in20.2.4.1also applies to AsyncFunction instances.
27.7.4.2 name
The specification for the"name"property of Function instances given in20.2.4.2also applies to AsyncFunction instances.
The abstract operation AsyncFunctionStart takes arguments promiseCapability (aPromiseCapability Record) and asyncFunctionBody. It performs the following steps when called:
Assert: result is anormal completionwith a value ofundefined. The possible sources of completion values areAwaitor, if the async function doesn't await anything, step4.gabove.
Return.
28 Reflection
28.1 The Reflect Object
The Reflect object:
is %Reflect%.
is the initial value of the"Reflect"property of theglobal object.
does not have a"prototype"property because Proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
has the following properties:
28.2.2.1 Proxy.revocable ( target, handler )
The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:
A Module Namespace Object is amodule namespace exotic objectthat provides runtime property-based access to a module's exported bindings. There is noconstructorfunction for Module Namespace Objects. Instead, such an object is created for each module that is imported by anImportDeclarationthat contains aNameSpaceImport.
In addition to the properties specified in10.4.6each Module Namespace Object has the following own property:
28.3.1 @@toStringTag
The initial value of the@@toStringTagproperty is the String value"Module".
This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings ofShared Data Blockevents, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from eachagent. When the program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced byabstract operationson SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by theabstract operationson SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of this specification. The nondeterministic introduction of events byabstract operationsis the interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the memory model. The semantics of these events is defined by considering graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular event graph is allowed or disallowed.
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in anagent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemoryRecord.
An abstract closure that returns a modifiedListof byte values from a readListof byte values and [[Payload]].
These events are introduced byabstract operationsor by methods on the Atomics object.
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.
In addition toShared Data Blockand Synchronize events, there arehost-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.
Note 2
Examples ofhost-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from oneagentto another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within theagent clustervia channels other than shared memory. It is assumed those events are appended toagent-orderduring evaluation like the other SharedArrayBuffer events.
Events are ordered within candidate executions by the relations defined below.
29.2 Agent Events Records
An Agent Events Record is aRecordwith the following fields.
Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
Let byte be bytesModified[payloadIndex].
Append byte to bytesRead.
Set byteLocation to byteLocation + 1.
Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduceReadModifyWriteSharedMemoryevents.
For acandidate executionexecution, execution.[[AgentOrder]] is aRelationon events that satisfies the following.
For each pair (E, D) inEventSet(execution), (E, D) is in execution.[[AgentOrder]] if there is someAgent Events Recordaer in execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E is before D inListorder of aer.[[EventList]].
Note
Eachagentintroduces events in a per-agentstrict total orderduring the evaluation. This is the union of those strict total orders.
If (E, D) is in execution.[[HostSynchronizesWith]], E and D are inHostEventSet(execution).
There is no cycle in the union of execution.[[HostSynchronizesWith]] and execution.[[AgentOrder]].
Note 1
For twohost-specific events E and D, E host-synchronizes-with D implies Ehappens-beforeD.
Note 2
The host-synchronizes-with relation allows thehostto provide additional synchronization mechanisms, such as postMessage between HTML workers.
29.6.5 synchronizes-with
For acandidate executionexecution, execution.[[SynchronizesWith]] is the leastRelationon events that satisfies the following.
For each pair (R, W) in execution.[[ReadsFrom]], (W, R) is in execution.[[SynchronizesWith]] if R.[[Order]] isSeqCst, W.[[Order]] isSeqCst, and R and W have equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], (S, Sw) is in execution.[[SynchronizesWith]].
For each pair (E, D) in execution.[[HostSynchronizesWith]], (E, D) is in execution.[[SynchronizesWith]].
Note 1
Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.
Note 2
Initevents do not participate in synchronizes-with, and are instead constrained directly byhappens-before.
Note 3
Not allSeqCstevents related byreads-fromare related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.
For acandidate executionexecution, execution.[[HappensBefore]] is the leastRelationon events that satisfies the following.
For each pair (E, D) in execution.[[AgentOrder]], (E, D) is in execution.[[HappensBefore]].
For each pair (E, D) in execution.[[SynchronizesWith]], (E, D) is in execution.[[HappensBefore]].
For each pair (E, D) inSharedDataBlockEventSet(execution), (E, D) is in execution.[[HappensBefore]] if E.[[Order]] isInitand E and D have overlapping ranges.
For each pair (E, D) inEventSet(execution), (E, D) is in execution.[[HappensBefore]] if there is an event F such that the pairs (E, F) and (F, D) are in execution.[[HappensBefore]].
Note
Because happens-before is a superset ofagent-order, candidate executions are consistent with the single-thread evaluation semantics of ECMAScript.
29.7 Properties of Valid Executions
29.7.1 Valid Chosen Reads
Acandidate executionexecution has valid chosen reads if the following algorithm returnstrue.
If there is aWriteSharedMemoryorReadModifyWriteSharedMemoryevent V that has byteLocation in its range such that the pairs (W, V) and (V, R) are in execution.[[HappensBefore]], then
Returnfalse.
Set byteLocation to byteLocation + 1.
Returntrue.
29.7.3 Tear Free Reads
Acandidate executionexecution has tear free reads if the following algorithm returnstrue.
Assert: The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
For each event W such that (R, W) is in execution.[[ReadsFrom]] and W.[[NoTear]] istrue, do
If R and W have equal ranges, and there is an event V such that V and W have equal ranges, V.[[NoTear]] istrue, W is not V, and (R, V) is in execution.[[ReadsFrom]], then
Returnfalse.
Returntrue.
Note
An event's [[NoTear]] field istruewhen that event was introduced via accessing anintegerTypedArray, andfalsewhen introduced via accessing a floating point TypedArray or DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned fashion via anintegerTypedArray, a single write event on that range must "win" when in a data race with other write events with equal ranges. More precisely, this requirement says an aligned read event cannot read a value composed of bytes from multiple, different write events all with equal ranges. It is possible, however, for an aligned read event to read from multiple write events with overlapping ranges.
For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.
For each pair (R, W) in execution.[[ReadsFrom]], there is noWriteSharedMemoryorReadModifyWriteSharedMemoryevent V inSharedDataBlockEventSet(execution) such that V.[[Order]] isSeqCst, the pairs (W, V) and (V, R) are in memory-order, and any of the following conditions are true.
The pair (W, R) is in execution.[[SynchronizesWith]], and V and R have equal ranges.
The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]] isSeqCst, and W and V have equal ranges.
The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]] isSeqCst, and V and R have equal ranges.
Note 1
This clause additionally constrainsSeqCstevents on equal ranges.
This clause together with the forward progress guarantee on agents ensure the liveness condition thatSeqCstwrites become visible toSeqCstreads with equal range in finite time.
Acandidate executionhas sequentially consistent atomics if a memory-order exists.
Note 3
While memory-order includes all events inEventSet(execution), those that are not constrained byhappens-beforeorsynchronizes-withare allowed to occur anywhere in the order.
29.7.5 Valid Executions
Acandidate executionexecution is a valid execution (or simply an execution) if all of the following are true.
If either (E, D) or (D, E) is in execution.[[ReadsFrom]], then
Returntrue.
Returnfalse.
29.9 Data Races
For an execution execution, two events E and D inSharedDataBlockEventSet(execution) are in a data race if the following algorithm returnstrue.
If E and D are in a race in execution, then
If E.[[Order]] is notSeqCstor D.[[Order]] is notSeqCst, then
Returntrue.
If E and D have overlapping ranges, then
Returntrue.
Returnfalse.
29.10 Data Race Freedom
An execution execution is data race free if there are no two events inSharedDataBlockEventSet(execution) that are in a data race.
A program is data race free if all its executions are data race free.
Thememory modelguarantees sequential consistency of all events for data race free programs.
29.11 Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of eachagentare interleaved with each other. For data race free programs, it is not necessary to understand the details of thememory model. The details are unlikely to build intuition that will help one to better write ECMAScript.
More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.
It is desirable to allow most program transformations that are valid in a single-agentsetting in a multi-agentsetting, to ensure that the performance of eachagentin a multi-agentprogram is as good as it would be in a single-agentsetting. Frequently these transformations are hard to judge. We outline some rules about program transformations that are intended to be taken as normative (in that they are implied by thememory modelor stronger than what thememory modelimplies) but which are likely not exhaustive. These rules are intended to apply to program transformations that precede the introductions of the events that make up theagent-order.
Let an agent-order slice be the subset of theagent-orderpertaining to a singleagent.
Let possible read values of a read event be the set of all values ofValueOfReadEventfor that event across all valid executions.
Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.
Atomics are carved in stone: Program transformations must not cause theSeqCstevents in an agent-order slice to be reordered with itsUnorderedoperations, nor itsSeqCstoperations to be reordered with each other, nor may a program transformation remove aSeqCstoperation from theagent-order.
(In practice, the prohibition on reorderings forces a compiler to assume that everySeqCstoperation is a synchronization and included in the finalmemory-order, which it would usually have to assume anyway in the absence of inter-agentprogram analysis. It also forces the compiler to assume that every call where the callee's effects on thememory-orderare unknown may containSeqCstoperations.)
Reads must be stable: Any given shared memory read must only observe a single value in an execution.
(For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by anotheragentafter the read.)
Possible read values must be nonempty: Program transformations cannot cause the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have force inmemory modelinsofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between twoSeqCstoperations, but the transformation may not remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.
Specifically, thememory modelis intended to allow code generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: thememory modeleffectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences, which thememory modeldoes not actually guarantee.
A number of local improvements to those basic patterns are also intended to be legal:
There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScripthostis a web browser. The content of this annex is normative but optional if the ECMAScripthostis not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.
B.1 Additional Syntax
B.1.1 Numeric Literals
The syntax and semantics of12.8.3is extended as follows except that this extension is not allowed forstrict mode code:
It is possible for string literals to precede aUse Strict Directivethat places the enclosing code instrict mode, and implementations must take care to not use this extended definition ofEscapeSequencewith such literals. For example, attempting to parse the following source text must fail:
The syntax of22.2.1is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on thegoal symbol.
Atom (22.2.2.8) evaluation rules for theAtomproductions except forAtom::PatternCharacterare also used for theExtendedAtomproductions, but withExtendedAtomsubstituted forAtom. The following evaluation rules, with parameter direction, are also added:
The productionExtendedAtom::\[lookahead =c]evaluates as follows:
Let A be the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
The escape function is a property of theglobal object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2 unescape ( string )
The unescape function is a property of theglobal object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.
The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:
B.2.2 Additional Properties of the String.prototype Object
B.2.2.1 String.prototype.substr ( start, length )
The substr method takes two arguments, start and length, and returns asubstringof the result of converting thethisvalue to a String, starting from index start and running for length code units (or through the end of the String if length isundefined). If start is negative, it is treated assourceLength + startwhere sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
The substr function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.2.2 String.prototype.anchor ( name )
When the anchor method is called with argument name, the following steps are taken:
B.2.2.2.1 CreateHTML ( string, tag, attribute, value )
The abstract operation CreateHTML takes arguments string, tag (a String), attribute (a String), and value. It performs the following steps when called:
Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence""".
The property"trimStart"is preferred. The"trimLeft"property is provided principally for compatibility with old code. It is recommended that the"trimStart"property be used in new ECMAScript code.
The initial value of the"trimLeft"property is the samefunction objectas the initial value of the String.prototype.trimStart property.
B.2.2.16 String.prototype.trimRight ( )
Note
The property"trimEnd"is preferred. The"trimRight"property is provided principally for compatibility with old code. It is recommended that the"trimEnd"property be used in new ECMAScript code.
The initial value of the"trimRight"property is the samefunction objectas the initial value of the String.prototype.trimEnd property.
B.2.3 Additional Properties of the Date.prototype Object
B.2.3.1 Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
When the getYear method is called with no arguments, the following steps are taken:
Return the value of the [[DateValue]] internal slot ofthis Date object.
B.2.3.3 Date.prototype.toGMTString ( )
Note
The toUTCString method is preferred. The toGMTString method is provided principally for compatibility with old code.
Thefunction objectthat is the initial value of Date.prototype.toGMTString is the samefunction objectthat is the initial value of Date.prototype.toUTCString.
B.2.4 Additional Properties of the RegExp.prototype Object
The compile method completely reinitializes thethisvalue RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
B.3 Other Additional Features
B.3.1 Labelled Function Declarations
Prior to ECMAScript 2015, the specification ofLabelledStatementdid not allow for the association of a statement label with aFunctionDeclaration. However, a labelledFunctionDeclarationwas an allowable extension fornon-strict codeand most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar production forLabelledStatementpermits use ofFunctionDeclarationas aLabelledItembut14.13.1includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of thehighlightedtext:
B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of aFunctionDeclarationas an element of aBlockstatement'sStatementList. However, support for that form ofFunctionDeclarationwas an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that usesBlocklevel function declarations is only portable among browser implementation if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:
A function is declared and only referenced within a single block
One or moreFunctionDeclarations whoseBindingIdentifieris the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock.
No other declaration of f that is not a var declaration occurs within the function code of g
A function is declared and possibly used within a singleBlockbut also referenced by an inner function definition that is not contained within that sameBlock.
One or moreFunctionDeclarations whoseBindingIdentifieris the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as anIdentifierReferencewithin another function h that is nested within g and no other declaration of f shadows the references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
One or moreFunctionDeclarationwhoseBindingIdentifieris the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as anIdentifierReferencewithin the function code of g that lexically follows theBlockcontaining the declaration of f.
The first use case is interoperable with the semantics ofBlocklevel function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses10,14, and15.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause10, clause15, clause19.2.1and clause16.1.7semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains aFunctionDeclarationfor which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create anearly error, a warning message should not be produced.
B.3.2.1 Changes to FunctionDeclarationInstantiation
It is a Syntax Error if theLexicallyDeclaredNamesofStatementListcontains any duplicate entries, unless the source code matching this production is notstrict mode codeand the duplicate entries are only bound by FunctionDeclarations.
It is a Syntax Error if theLexicallyDeclaredNamesofCaseBlockcontains any duplicate entries, unless the source code matching this production is notstrict mode codeand the duplicate entries are only bound by FunctionDeclarations.
This production only applies when parsingnon-strict code. Code matching this production is processed as if each matching occurrence ofFunctionDeclaration[?Yield, ?Await, ~Default]was the soleStatementListItemof aBlockStatementoccupying that position in the source code. The semantics of such a syntheticBlockStatementincludes the web legacy compatibility semantics specified inB.3.2.
B.3.4 VariableStatements in Catch Blocks
The content of subclause14.15.1is replaced with the following:
TheBlockof aCatchclause may contain var declarations that bind a name that is also bound by theCatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by theCatchParameterand hence theInitializerfor such var declarations will assign to the corresponding catch parameter rather than the var binding.
This modified behaviour also applies to var and function declarations introduced bydirect evalcalls contained within theBlockof aCatchclause. This change is accomplished by modifying the algorithm of19.2.1.3as follows:
Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is ahost-definedexotic objectwith this slot that exists for web compatibility purposes. There are no other known examples of this type of object and implementations should not create any with the exception of document.all.
B.3.6.1 Changes to ToBoolean
The result column inTable 13for an argument type of Object is replaced with the following algorithm:
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in theglobal object. When a simple assignment occurs withinstrict mode code, itsLeftHandSideExpressionmust not evaluate to an unresolvable Reference. If it does aReferenceErrorexception is thrown (6.2.4.6). TheLeftHandSideExpressionalso may not be a reference to adata propertywith the attribute value { [[Writable]]:false}, to anaccessor propertywith the attribute value { [[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal slot has the valuefalse. In these cases a TypeError exception is thrown (13.15).
Arguments objects for strict functions define a non-configurableaccessor property"callee"which throws aTypeErrorexception on access (10.4.4.6).
Arguments objects for strict functions do not dynamically share theirarray-indexedproperty values with the corresponding formal parameter bindings of their functions. (10.4.4).
For strict functions, if an arguments object is created the binding of the local identifier arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (10.2.11).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (19.2.1).
Ifthisis evaluated withinstrict mode code, then thethisvalue is not coerced to an object. Athisvalue ofundefinedornullis not converted to theglobal objectand primitive values are not converted to wrapper objects. Thethisvalue passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passedthisvalue to an object (10.2.1.2,20.2.3.1,20.2.3.3).
When a delete operator occurs withinstrict mode code, aSyntaxErroris thrown if itsUnaryExpressionis a direct reference to a variable, function argument, or function name (13.5.1.1).
When a delete operator occurs withinstrict mode code, aTypeErroris thrown if the property to be deleted has the attribute { [[Configurable]]:false} or otherwise cannot be deleted (13.5.1.2).
An implementation may not extend, beyond that defined in this specification, the meanings within strict functions of properties named"caller"or"arguments"of function instances.
Any of the essential internal methods inTable 6for anyexotic objectnot specified within this specification.
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in17.1.
E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
9.1.1.4.15-9.1.1.4.18Edition 5 and 5.1 used a property existence test to determine whether aglobal objectproperty corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.
10.4.2.1: The 5th Edition moved the capture of the current array length prior to theintegerconversion of thearray indexor new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.
21.4.1.14: Previous editions permitted theTimeClipabstract operation to return either+0𝔽 or-0𝔽 as the representation of a 0time value. ECMAScript 2015 specifies that+0𝔽 always returned. This means that for ECMAScript 2015 thetime valueof a Date object is never observably-0𝔽 and methods that return time values never return-0𝔽.
21.4.1.15: If a UTC offset representation is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as"z".
21.4.4.36: If the year cannot be represented using the Date Time String Format specified in21.4.1.15a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
21.4.4.41: Previous editions did not specify the value returned by Date.prototype.toString whenthis time valueisNaN. ECMAScript 2015 specifies the result to be the String value"Invalid Date".
22.2.3.1,22.2.3.2.5: Any LineTerminator code points in the value of the"source"property of a RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of /.
22.2.5.7,22.2.5.10: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by one only if the pattern matched the empty String.
23.1.3.27,23.1.3.27.1: Previous editions did not specify how aNaNvalue returned by a comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if+0𝔽 was returned from the comparefn. ECMAScript 2015 also specifies thatToNumberis applied to the result returned by a comparefn. In previous editions, the effect of a comparefn result that is not aNumber valuewasimplementation-defined. In practice, implementations callToNumber.
F Additions and Changes That Introduce Incompatibilities with Prior Editions
6.2.4: In ECMAScript 2015, Function calls are not allowed to return aReference Record.
9.3: In ECMAScript 2018, Template objects are canonicalized based onParse Node(source location), instead of across all occurrences of that template literal or tagged template in aRealmin previous editions.
12.2: In ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example, "\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode standard.
12.6: In ECMAScript 2015, the valid code points for anIdentifierNameare specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the validIdentifierNameorIdentifiercode points were specified by enumerating various Unicode code point categories.
12.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.
13.2.5.1: In ECMAScript 2015, it is no longer anearly errorto have duplicate property names in Object Initializers.
13.15.1: In ECMAScript 2015,strict mode codecontaining an assignment to an immutable binding such as the function name of aFunctionExpressiondoes not produce anearly error. Instead it produces a runtime error.
14.2: In ECMAScript 2015, aStatementListbeginning with the token let followed by the input elementsLineTerminatorthenIdentifieris the start of aLexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before theIdentifierinput element.
14.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of aLexicalDeclaration. In previous editions such a token sequence would be the start of anExpression.
14.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of aForDeclaration. In previous editions such a token sequence would be the start of anLeftHandSideExpression.
14.7: Prior to ECMAScript 2015, an initialization expression could appear as part of theVariableDeclarationthat precedes the inkeyword. In ECMAScript 2015, theForBindingin that same position does not allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only innon-strict code.
14.15: In ECMAScript 2015, it is anearly errorfor aCatchclause to contain a var declaration for the sameIdentifierthat appears as theCatchclause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration'sInitializervalue would be assigned to theCatchparameter.
14.15,19.2.1.3: In ECMAScript 2015, a runtimeSyntaxErroris thrown if aCatchclause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the sameIdentifierthat appears as theCatchclause parameter.
15.4.5In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in anObjectLiteralare notconstructorfunctions and they do not have a"prototype"own property. In the previous edition, they were constructors and had a"prototype"property.
20.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.8: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.10: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.12: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.14: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.15: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.16: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.17: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.18: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.1.2.20: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.
20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to%Function.prototype%.
20.2.4.1: In ECMAScript 2015, the"length"property of function instances is configurable. In previous editions it was non-configurable.
21.4.4In ECMAScript 2015, theDate prototype objectis not a Date instance. In previous editions it was a Date instance whose TimeValue wasNaN.
22.1.3.10In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.26and22.1.3.28In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode.
22.1.3.29In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exist outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.
22.2.3.1In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is notundefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions aTypeErrorexception was thrown when pattern was a RegExp instance and flags was notundefined.
22.2.5In ECMAScript 2015, theRegExp prototype objectis not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty String.
22.2.5In ECMAScript 2015,"source","global","ignoreCase", and"multiline"are accessor properties defined on theRegExp prototype object. In previous editions they were data properties defined on RegExp instances.
25.4.13: In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent confusion with Atomics.wait.
27.1.4.4,27.6.3.6: In ECMAScript 2019, the number of Jobs enqueued by await was reduced, which could create an observable difference in resolution order between a then() call and an await expression.
G Colophon
This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.
H Bibliography
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2019)Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262 specification.
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